Girdling as an expression is used in two ways regarding trees - first is girdling of the trunk deliberately, above ground, often done to kill trees, or inadvertently by leaving ties in place too long; the second is when the roots, or a root, encircle the trunk below ground and gradually strangle it, and that's the form of girdling you're worried about in your situation. It's much better to try to correct any possible problems now, as you've so recently planted, rather than waiting to see symptoms in a few years.
Link below gives quite a lot of information as to how to plant a tree to try to avoid this form of girdling - in your current circumstances, you've said the soil is heavy clay, and you only dug an area twice the size of the rootball. In the information below, you will see that the most critical thing to make sure of to avoid girdling is the level at which the tree is planted, that is, the rootball must not be planted too deeply. If you have planted it too deeply, you may need to remove it and replant a bit higher.
It will likely pay you to immediately rototill (or just dig) the surrounding area where you've planted the tree to a width of 3 or 4 times the size of the existing rootball, given that your soil is heavy clay, possibly compacted, and may be very wet at times. This method is described under Section 2, Modification for Compacted Soils, but its probably worth scanning through the whole document anyway, to check whether you needed to leave the rootball slightly proud of the soil. If you feel you have planted too deeply, its only been a month since you planted and is therefore not too late to take it out and adjust the hole, making it a saucer shape rather than straight sided, as demonstrated in the article. Wider rototilling of the area is best done once the tree is planted anyway.
Having looked again at your post about this tree back in September, if that's an allotment you've planted it in, it looks as if you have previously dug all the soil over at some point in the last year or months, so I don't think you have a problem unless you planted the tree too low...