Musical instrument in grip on top of case (7-3)
I believe the answer is:
squeeze-box
'musical instrument' is the definition.
(squeeze box is a type of musical instrument)
'grip on top of case' is the wordplay.
'grip' becomes 'squeeze' (squeezing is a kind of gripping).
'on top of' is a charade indicator (letters next to each other).
'case' becomes 'box' (boxing is a kind of casing).
'squeeze'+'box'='SQUEEZE-BOX'
'in' acts as a link.
(Other definitions for squeeze-box that I've seen before include "Informal name for accordion" , "Concertina - accordion" .)