Look, it's Mark! (4)
I believe the answer is:
spot
I believe this clue is a double definition.
'look' is the first definition.
(thesaurus)
'mark' is the second definition.
(spotting is a kind of marking)
'it's' is the link.
(Other definitions for spot that I've seen before include "Notice; skin blemish" , "Predicament" , "Catch sight of - pimple" , "[NIGHTCLUB]" , "Pickle" .)