Balance one in position (5)
I believe the answer is:
poise
'balance' is the definition.
(I've seen this before)
'one in position' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'position' becomes 'pose' (posing is a kind of positioning**).
'i' put within 'pose' is 'POISE'.
(Other definitions for poise that I've seen before include "Elegance" , "cool" , "Physical balance" , "Dignity; equilibrium" , "Coolness and composure under strain" .)