Girl in a pool game (4)
I believe the answer is:
olga
'girl' is the definition.
'in a pool game' is the wordplay.
'in' indicates a hidden word.
'OLGA' can be found hidden inside 'pool game'.
(Other definitions for olga that I've seen before include "Lady" , "Russian girl's name, anagram of ..." , "Could be Russian girl" , "Slavic girl's name" .)