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