Who's keen to play Partridge Queen? (5)
I believe the answer is:
gamer
'who's keen to play' is the definition.
(someone who plays games)
'partridge queen?' is the wordplay.
'partridge' becomes 'game' (game bird).
'queen?' becomes 'r' (abbreviation for regina, e.g. in ER ).
'game'+'r'='GAMER'
(Other definitions for gamer that I've seen before include "One having fun with computer" , "One plays" , "Keen computer player" , "One role-playing on computer" , "More spirited" .)