street fighter vs combatant

street fighter and combatant both are nouns.

Word	Noun	Adjective	Verb	Adverb
street fighter	Yes	No	No	No
combatant	Yes	Yes	No	No

As nouns, combatant is a hypernym of street fighter; that is, combatant is a word with a broader meaning than street fighter:

street fighter: someone who learned to fight in the streets rather than being formally trained in the sport of boxing
combatant: someone who fights (or is fighting)

Other hypernyms of street fighter include battler, belligerent, fighter, scrapper.

street fighter (noun)	combatant (noun)
a contestant who is very aggressive and willing to use underhand methods	someone who fights (or is fighting)
someone who learned to fight in the streets rather than being formally trained in the sport of boxing

street fighter (adjective)	combatant (adjective)
	engaging in or ready for combat