Wrote music that’s not frantic (8)
I believe the answer is:
composed
This clue is a double definition.
'wrote music' is the first definition.
('compose' can be a synonym of 'write music')
'not frantic' is the second definition.
'that's' is the link.
(Other definitions for composed that I've seen before include "cool" , "Written" , "Wrote music" , "as score must be" , "Calm and collected" .)