Pretty girl holding a gun must be game (9)
I believe the answer is:
bagatelle
'game' is the definition.
(I know that bagatelle is a type of table game)
'pretty girl holding a gun' is the wordplay.
'pretty girl' becomes 'belle' (I've seen this before).
'holding' indicates putting letters inside.
'gun' becomes 'gat' (US informal term for a gun).
'a'+'gat'='agat'
'belle' enclosing 'agat' is 'BAGATELLE'.
'must be' is the link.
(Other definitions for bagatelle that I've seen before include "Trivial matter" , "trifling amount" , "Game; piece of music" , "Unimportant thing" , "Trifle; pinball game" .)