A watchful and bruising opponent? (6)
I believe the answer is:
marker
'opponent?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for marker that I've seen before include "Visible point to aim for" , "Writing implement" , "Felt-tip pen; scorer" , "Buoy" , "Test official" .)