Composer on doctrine about musical form (5)
I believe the answer is:
rondo
'musical form' is the definition.
(I know that rondo is a musical form)
'composer on doctrine about' is the wordplay.
'about' says the answer is hidden in the clue (the clue goes around or about the answer).
'RONDO' can be found hidden inside 'composer on doctrine'.
(Other definitions for rondo that I've seen before include "part of a sonata" , "Musical form with recurrent theme" , "Piece written" , "Piece in the form of a sonata's last movement, say" , "Musical form with recurring theme" .)