His opponent, presumably, is a marked man (7)
I believe the answer is:
bruiser
'his opponent presumably is a marked' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'man' is the wordplay.
I cannot really understand how this works, but
'man' could be 'bruis' and 'bruis' is found in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for bruiser that I've seen before include "Tough, aggressive person" , "Big aggressive man" , "tough guy" , "Yobbo" , "Fighter -- bully" .)