What Gershwin did is cool (8)
I believe the answer is:
composed
I believe this clue is a double definition.
'what gershwin did' is the first definition.
(I've seen this before)
'cool' is the second definition.
(similar in meaning)
'is' is the link.
(Other definitions for composed that I've seen before include "What poet did" , "with aplomb" , "wrote notes" , "Still - contrived" , "Wrote music" .)