I have an indoor Croton. It keeps growing vertically. Is it possible to trim to encourage horizontal bushier growth?
You can remove up to a third of the length of that stem, but I'm afraid, given its situation, it's unlikely to improve the situation much, because its not receiving enough bright light/sunlight all over. It really needs complete exposure, that is, the whole plant from top to bottom needs to be in bright light, and yours isn't, which is likely why it's grown in such a manner.
You can still try removing up to a third and see what happens, but really, it needs to be sat on a table or something to raise it up in front of the window - it will also do best in a southfacing window.
I'd suggest an alternative option if you're feeling adventurous.
Worth keeping in mind, that the Croton is a durable plant in tropical climates, although can be less so in colder climates and therefore may require a greater level of care.
Regardless, from the photo you've provided, I'd suggest your plant is healthy.
As mentioned by @bamboo the timing is critical. Complete the process just before spring and before the period of the plant's annual growth spurt. If you want to be certain, look for the beginnings of new growth from lateral / axillary buds... see diagram from Wikipedia...
I've had success with much harder pruning, removing one half to two thirds of the primary trunk. I'd suggest you consider that for a couple of reasons.
Firstly it's going to encourage much stronger growth from remaining fewer lateral buds in springtime.
Secondly it's going to provide a greater amount of plant material for the adventurous part...
Purchase a larger pot with a greater diameter across the top. Relocate the plant into this pot now and make certain you provide it with some liquid tonic such as the seaweed emulsion type of products that are as much about maintaining healthy soil as they are about promoting plant growth. Don't overwater. Tonic every two weeks in summer to six or eight weeks in winter.
Buy some rooting hormone liquid.
When it comes time to prune, consider using the piece of plant material left over from the pruning to "strike" new young plants.
With the remaining piece of trunk, cut into sections such that each section has three to four pairs of leaves and pairs of lateral buds.
From each piece, remove all but the top pair of leaves. Then cut those top leaves so that only one quarter the surface area remains - that is - cut each leaf so that only a quarter of the leaf remains. Scratch out the bottom pair of lateral buds (if three pairs, or bottom two pairs if four pairs). Scratch the bark down one side of stem from the lateral buds to the cut. Dip into rooting hormone liquid. Place each piece into a small pot with fine potting mix. Locate in a convenient spot, preferably with morning sun, so you see it every day and therefore remember to keep the soil moist.
The original top leaves may die and fall off, but be patient. Wait a few more weeks and if you're successful, you could strike another few plants.
If they survive to a year old, you can plant them in with the parent plant! Then you'll have a much denser plant as the original plant will have branched out more, but then you'll also have additional plants.
Only if you're feeling adventurous.
Good luck and happy growing :)