Contestant who gives his opponent marks (7)
I believe the answer is:
bruiser
This clue might be a double definition.
'contestant' is the first definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'gives his opponent marks' is the second definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'who' is the link.
Can you help me to learn more?
(Other definitions for bruiser that I've seen before include "Big aggressive man" , "Informally, a large tough person, maybe a boxer" , "Large tough man, inflicts flesh discolourations" , "Fighter - rues rib (anag)" , "Tough, aggressive person" .)