Write as music (6)
I believe the answer is:
notate
'write as music' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for notate that I've seen before include "Record in bars" , "Write using signs or symbols as in music etc" , "Put down in symbols" , "Add runs to score, possibly" , "Record, say" .)