Hamilton antagonist (4)
I believe the answer is:
burr
'hamilton antagonist' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for burr that I've seen before include "Clinging seed-case; whetstone" , "Rough cut edge; West Country accent" , "Soft regional accent" , "Accent, maybe Northumbrian or West Country" , "Rotary file - feature of some regional accents" .)