Room to love instrument (5)
I believe the answer is:
cello
'instrument' is the definition.
(cello is a kind of instrument)
'room to love' is the wordplay.
'room' becomes 'cell' (cell is a kind of room).
'to love' becomes 'o' (love means zero in tennis).
'cell'+'o'='CELLO'
(Other definitions for cello that I've seen before include "Bass stringed instrument" , "Bowed musical instrument" , "Large stringed instrument" , "Lloyd Webber's instrument" , "Viola's bigger brother?" .)