Use penny Oscar kept in wallet (7)
I believe the answer is:
purpose
'use' is the definition.
(something's purpose is its use or function)
'penny oscar kept in wallet' is the wordplay.
'penny' becomes 'p' (short for pence).
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'kept in' means one lot of letters goes inside another.
'wallet' becomes 'purse' (I've seen this before).
'p'+'o'='po'
'po' put inside 'purse' is 'PURPOSE'.
(Other definitions for purpose that I've seen before include "Resolve; reason" , "Function" , "Reason for existence" , "Resolve, determination" , "Resolve, objective" .)