Room next to old instrument (5)
I believe the answer is:
cello
'instrument' is the definition.
(cello is a kind of instrument)
'room next to old' is the wordplay.
'room' becomes 'cell' (cell is a kind of room).
'next to' means one lot of letters go next to another.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'cell'+'o'='CELLO'
(Other definitions for cello that I've seen before include "Tortelier's instrument" , "Played by Rostropovich" , "Musical instrument like large violin held between knees" , "has strings attached" , "big fiddle?" .)