I am working on a composition of mine. When I transposed an already high section on the Cello to a different key it ended up having an A5 in it.
Is this possible to play for a good Cello player?
It is easy to play A5 (or, indeed, even A6) on cello as a natural flageolett. However (even if the airy sound of a flageolett is ok for your application), it won't help with any tones merely close to A5. So if that's just the very tip of a melodic line, you'll need to think of a position where also the other notes are feasible as non-flageoletts. The 11th position might be suitable, it has the thumb resting nicely on the third harmonic (A4 on the D string) and the A5 sitting quite reachable for the 3rd finger. This position works well for the keys of G to A; for ♭-keys it's not so nice (for instance the F5 lies uncomfortably close to the thumb).
So – it is certainly possible, though how well it can work out depends on the exact context. In general, please try to think about how passages can be played, not just if it's possible.