Musical instrument in small room, old (5)
I believe the answer is:
cello
'musical instrument' is the definition.
(cello is a kind of musical instrument)
'small room old' is the wordplay.
'small room' becomes 'cell' (eg a prison cell).
'old' becomes 'o' (abbreviation).
'cell'+'o'='CELLO'
'in' is the link.
(Other definitions for cello that I've seen before include "Tortelier's instrument" , "Bass stringed instrument" , "Large stringed instrument" , "Orchestral instrument played between the legs" , "Viola's bigger brother?" .)