Players sit: boos break out (7)
I believe the answer is:
oboists
'players' is the definition.
(oboist is a kind of player)
'sit boos break out' is the wordplay.
'break out' indicates anagramming the letters.
'sit'+'boos'='sitboos'
'sitboos' with letters rearranged gives 'OBOISTS'.
(Other definitions for oboists that I've seen before include "Woodwind players" , "10 across group" , "Musicians" .)