Band playing Hart score (9)
I believe the answer is:
orchestra
'band' is the definition.
(I've seen this before)
'playing hart score' is the wordplay.
'playing' is an anagram indicator.
'hart'+'score'='hartscore'
'hartscore' with letters rearranged gives 'ORCHESTRA'.
(Other definitions for orchestra that I've seen before include "many players" , "Group of musicians found on cart-horse" , "Group of musical instrumentalists" , "Ensemble of players" , "A number of musicians" .)