Mark and Ian may be on the street (5)
I believe the answer is:
stain
'mark' is the definition.
(synonyms)
'ian may be on the street' is the wordplay.
'may be' indicates anagramming the letters.
'on' is an insertion indicator (as in clothing 'on' a person).
'the street' becomes 'st' (abbreviation).
'ian' anagrammed gives 'ain'.
'ain' enclosing 'st' is 'STAIN'.
'and' acts as a link.
(Other definitions for stain that I've seen before include "Discolouration" , "Taint" , "Discoloured patch" , "Blemish; colour (wood)" , "Satin (anag.)" .)