Room over an instrument (5)
I believe the answer is:
cello
'an instrument' is the definition.
(cello is a kind of instrument)
'room over' is the wordplay.
'room' becomes 'cell' (cell is a kind of room).
'over' becomes 'o' (cricket abbreviation).
'cell'+'o'='CELLO'
(Other definitions for cello that I've seen before include "10 [INSTRUMENT]" , "Musical instrument like large violin held between knees" , "Music-maker" , "It's bigger than a viola, but smaller than a double bass" , "Casals' instrument" .)