Room to put old instrument (5)
I believe the answer is:
cello
'instrument' is the definition.
(cello is a kind of instrument)
'room to put old' is the wordplay.
'room' becomes 'cell' (cell is a kind of room).
'to put' is a charade indicator (letters next to each other).
'old' becomes 'o' (abbreviation).
'cell'+'o'='CELLO'
(Other definitions for cello that I've seen before include "music produced by this?" , "Bass stringed instrument" , "Tortelier's instrument" , "big fiddle?" , "String quartet instrument" .)