Opera if not comic might be grand (10)
I believe the answer is:
pianoforte
'might be grand' is the definition.
(I've seen this before)
'opera if not comic' is the wordplay.
'comic' indicates anagramming the letters (an unusual or comical spelling).
'opera'+'if'+'not'='operaifnot'
'operaifnot' with letters rearranged gives 'PIANOFORTE'.
(Other definitions for pianoforte that I've seen before include "might be grand" , "Keyboard instrument" , "Joanna" .)