Muck's partner (4)
I believe the answer is:
mire
'muck's partner' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mire that I've seen before include "Bog - muck" , "Mud - bog" , "Cause to get stuck (in soft mud?)" , "Soft mud or dirt" , "Morass" .)