I sang carol composed for English horn (3,7)
I believe the answer is:
cor anglais
'english horn' is the definition.
(synonyms)
'i sang carol composed' is the wordplay.
'composed' indicates anagramming the letters.
'i'+'sang'+'carol'='isangcarol'
'isangcarol' is an anagram of 'COR ANGLAIS'.
'for' is the link.
(Other definitions for cor anglais that I've seen before include "MUSICAL INSTRUMENT" , "Woodwind instrument" , "horn?" , "belonging to a broken-down 27 [CARTHORSE]" , "Orchestra member rarely" .)