Composer finds key harms composition (6)
I believe the answer is:
brahms
'composer' is the definition.
(I know that Brahms is a composer)
'key harms composition' is the wordplay.
'key' becomes 'b' (eg B major, musical key).
'composition' indicates anagramming the letters (letters arranged or composed in a different order).
'harms' is an anagram of 'rahms'.
'b'+'rahms'='BRAHMS'
'finds' is the link.
(Other definitions for brahms that I've seen before include "Johannes -, nineteenth century German composer" , "Johannes . . . . . ., German composer of famous lullaby" , "Composer of a well-known lullaby published in 1868" , "Composer of A German Requiem" , "'Bah, Mrs, he's a composer (6)'" .)