With a postural fault, keep out of the ring (5)
I believe the answer is:
stoop
'with a postural fault keep' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'out of the ring' is the wordplay.
'out of' is an insertion indicator (some letters on the outside of others).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'ring' becomes 'soop' (I can't justify this - if you can you should give a lot more credence to this answer).
't' put into 'soop' is 'STOOP'.
Can you help me to learn more?
(Other definitions for stoop that I've seen before include "Condescend" , "Bent posture" , "Hunch" , "Deign" , "Sink" .)