Oppose sound of horse, perhaps (6)
I believe the answer is:
naysay
'oppose' is the definition.
(naysayers oppose something)
'sound of horse perhaps' is the wordplay.
'sound of horse' becomes 'nay' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'perhaps' becomes 'say' (both can mean 'for example').
'nay'+'say'='NAYSAY'
(Other definitions for naysay that I've seen before include "refusal" , "Deny" , "refuse" .)