Trifle with pretty girl carrying a pistol (9)
I believe the answer is:
bagatelle
'trifle' is the definition.
(thesaurus)
'pretty girl carrying a pistol' is the wordplay.
'pretty girl' becomes 'belle' (I've seen this before).
'carrying' is an insertion indicator.
'pistol' becomes 'gat' (gat is a kind of pistol).
'a'+'gat'='agat'
'belle' placed around 'agat' is 'BAGATELLE'.
'with' is the link.
(Other definitions for bagatelle that I've seen before include "Something trivial" , "Trivial matter" , "no serious work" , "game at the tables?" , "trifling amount" .)