Instrument found in small room by orchestra's leader (5)
I believe the answer is:
cello
'instrument' is the definition.
(cello is a kind of instrument)
'small room by orchestra's leader' is the wordplay.
'small room' becomes 'cell' (I've seen this before).
'by' is a charade indicator (letters next to each other).
'leader' indicates taking the first letters.
The first letter of 'orchestras' is 'o'.
'cell'+'o'='CELLO'
'found in' is the link.
(Other definitions for cello that I've seen before include "Musical instrument like large violin held between knees" , "Music-maker" , "Viola's bigger brother?" , "Lloyd Webber's instrument" , "Tortelier's instrument" .)