Little room, inside case, for musical instrument (5)
I believe the answer is:
cello
'musical instrument' is the definition.
(cello is a kind of musical instrument)
'little room inside case' is the wordplay.
'little room' becomes 'cell' (I've seen this before).
'inside' indicates the central letters.
'case' becomes 'cod' (cod is a kind of case).
The middle letter of 'cod' is 'o'.
'cell'+'o'='CELLO'
'for' is the link.
(Other definitions for cello that I've seen before include "It's bigger than a viola, but smaller than a double bass" , "Instrument in the violin family" , "Lloyd Webber's instrument" , "Musical instrument like large violin held between knees" , "Music-maker" .)