Opposition leader wheels in a musical instrument (7)
I believe the answer is:
ocarina
'musical instrument' is the definition.
(ocarina is a kind of musical instrument)
'opposition leader wheels in a' is the wordplay.
'leader' suggests taking the first letters.
'wheels' becomes 'car' (wheels can informally mean a car).
The initial letter of 'opposition' is 'o'.
'o'+'car'+'in'+'a'='OCARINA'
(Other definitions for ocarina that I've seen before include "dated instrument" , "Primitive wind instrument" , "Play a musical toy in the ICA, Nora" , "Bird-shaped flute" , "Egg-shaped type of flute" .)