Mark isn't a problem (5)
I believe the answer is:
stain
'mark' is the definition.
(synonyms)
'isn't a problem' is the wordplay.
'problem' indicates an anagram.
'isnt'+'a'='isnta'
'isnta' anagrammed gives 'STAIN'.
(Other definitions for stain that I've seen before include "Blotch on satin" , "Discolouring mark" , "Discoloured patch" , "Colour, dye" , "Discoloration" .)