Possibly sit and boo an instrumentalist (6)
I believe the answer is:
oboist
'an instrumentalist' is the definition.
(I know that oboist is a type of instrumentalist)
'possibly sit and boo' is the wordplay.
'possibly' indicates anagramming the letters.
'and' means one lot of letters go next to another.
'sit' after 'boo' is 'boosit'.
'boosit' with letters rearranged gives 'OBOIST'.
(Other definitions for oboist that I've seen before include "Musician who plays woodwind of treble pitch" , "Instrumentalist" , "Player of woodwind instrument of treble pitch" , "Wind player" , "orchestra member" .)