So a girl can play musical instrument! (3,7)
I believe the answer is:
cor anglais
'musical instrument' is the definition.
(cor anglais is a kind of musical instrument)
'so a girl can play' is the wordplay.
'play' indicates anagramming the letters.
'so'+'a'+'girl'+'can'='soagirlcan'
'soagirlcan' with letters rearranged gives 'COR ANGLAIS'.
(Other definitions for cor anglais that I've seen before include "Woodwind instrument" , "alto member of family" , "horn?" , "MUSICAL INSTRUMENT" , "Orchestra member rarely" .)