Small room leader of orchestra needed for instrument (5)
I believe the answer is:
cello
'for instrument' is the definition.
(cello is a kind of instrument)
'small room leader of orchestra needed' is the wordplay.
'small room' becomes 'cell' (eg a prison cell).
'leader of' says to take the initial letters.
'needed' is a charade indicator (letters next to each other).
The first letter of 'orchestra' is 'o'.
'cell'+'o'='CELLO'
(Other definitions for cello that I've seen before include "Casals' instrument" , "Bass stringed instrument" , "Large stringed instrument" , "Bowed musical instrument" , "Tortelier's instrument" .)