Room initially only for an instrument (5)
I believe the answer is:
cello
'for an instrument' is the definition.
(cello is a kind of instrument)
'room initially only' is the wordplay.
'room' becomes 'cell' (cell is a kind of room).
'initially' says to take the initial letters.
The first letter of 'only' is 'o'.
'cell'+'o'='CELLO'
(Other definitions for cello that I've seen before include "Tortelier's instrument" , "Jacqueline du Pre's instrument" , "Bowed musical instrument" , "Lloyd Webber's instrument" , "Music-maker" .)