Instrument for a girl (5)
I believe the answer is:
viola
'girl' is the definition.
'instrument for a' is the wordplay.
'instrument' becomes 'viol' (viol is a kind of instrument).
'for' says to put letters next to each other.
'viol'+'a'='VIOLA'
(Other definitions for viola that I've seen before include "Shakespearean character" , "part of Shakespeare" , "Shakespearean role" , "Pansy genus" , "Flower -- stringed instrument" .)