Girlfriend has case for musical instrument (7,3)
I believe the answer is:
squeeze box
'musical instrument' is the definition.
(squeeze box is a type of musical instrument)
'girlfriend has case' is the wordplay.
'girlfriend' becomes 'squeeze' (squeeze can mean a romantic partner).
'has' means one lot of letters go next to another.
'case' becomes 'box' (boxing is a kind of casing).
'squeeze'+'box'='SQUEEZE-BOX'
'for' is the link.
(Other definitions for squeeze box that I've seen before include "Music-maker" , "Concertina" , "musical instrument" , "producer of note" , "Accordion" .)