Stance to keep one's balance (5)
I believe the answer is:
poise
'balance' is the definition.
(I've seen this before)
'stance to keep one's' is the wordplay.
'stance' becomes 'pose'.
'to keep' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'pose' placed around 'i' is 'POISE'.
(Other definitions for poise that I've seen before include "Dignity and assurance of manner" , "Dignified and self-assured manner" , "Graceful and elegant bearing in a person" , "cool" , "Sang-froid" .)