Composer Gustav (6)
I believe the answer is:
mahler
'composer gustav' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mahler that I've seen before include "Gustav -, Austrian symphonist" , "Gustav . . . . . ., Austrian composer" , "Austrian symphonic composer, d. 1911" , "Interrupt; hang" , "Austrian composer from Harlem?" .)