Boundaries of jazz-rock expanded by an outstanding composer (7)
I believe the answer is:
janacek
'composer' is the definition.
(I've seen this before)
'boundaries of jazz-rock expanded by an outstanding' is the wordplay.
I cannot really see how this works, but
'an' could be 'a' and 'a' is found in the answer.
'outstanding' could be 'ace' (I've seen this before) and 'ace' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for janacek that I've seen before include "Czech composer known mainly for operas" , "Czech composer (The Cunning Little Vixen)" .)