Ballgame beauty packs a pistol (9)
I believe the answer is:
bagatelle
'ballgame' is the definition.
(game involving balls)
'beauty packs a pistol' is the wordplay.
'beauty' becomes 'belle' (I've seen this before).
'packs' is an insertion indicator (inserted letters are packed inside).
'pistol' becomes 'gat' (gat is a kind of pistol).
'a'+'gat'='agat'
'belle' enclosing 'agat' is 'BAGATELLE'.
(Other definitions for bagatelle that I've seen before include "Pinball game - something of little value" , "shooting game" , "Board-game, trifle" , "trifling amount" , "Trifling matter" .)