Mark isn't a convert (5)
I believe the answer is:
stain
'mark' is the definition.
(I know that mark can be written as stain)
'isn't a convert' is the wordplay.
'convert' indicates an anagram (I've seen 'converts' mean this).
'isnt'+'a'='isnta'
'isnta' is an anagram of 'STAIN'.
(Other definitions for stain that I've seen before include "Taint" , "Discoloured patch" , "Discolouration" , "Discoloured mark" , "Colour, dye" .)