Use prop when playing role (7)
I believe the answer is:
purpose
'role' is the definition.
(synonyms)
'use prop when playing' is the wordplay.
'when playing' is an anagram indicator.
'use'+'prop'='useprop'
'useprop' is an anagram of 'PURPOSE'.
(Other definitions for purpose that I've seen before include "Will - point" , "Resolve; reason" , "Aim, intention, design" , "Reason for existence" , "Resolve, determination" .)