Instrument held in one area? Unusual (10)
I believe the answer is:
kettledrum
'instrument' is the definition.
(kettledrum is a kind of instrument)
'held in one area? unusual' is the wordplay.
'held in one area?' becomes 'kettled' (as in kettling, method of crowd control).
'unusual' becomes 'rum' (I've seen this before).
'kettled'+'rum'='KETTLEDRUM'
(Other definitions for kettledrum that I've seen before include "Part of kitchen" , "One beaten by player" , "17 instrument" , "Orchestral instrument" , "thing to produce tattoo" .)