Model, keeping one's balance (5)
I believe the answer is:
poise
'balance' is the definition.
(I've seen this before)
'model keeping one's' is the wordplay.
'model' becomes 'pose' ('pose' can be a synonym of 'model'**).
'keeping' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'pose' placed around 'i' is 'POISE'.
(Other definitions for poise that I've seen before include "Grace; equilibrium" , "Balance; composure" , "Dignified and self-assured manner" , "cool" , "Self-confidence" .)