Room with old instrument (5)
I believe the answer is:
cello
'instrument' is the definition.
(cello is a kind of instrument)
'room with old' is the wordplay.
'room' becomes 'cell' (cell is a kind of room**).
'with' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'cell'+'o'='CELLO'
(Other definitions for cello that I've seen before include "Tortelier's instrument" , "Musical instrument like large violin held between knees" , "Orchestral instrument played between the legs" , "Large stringed instrument" , "Casals' instrument" .)