Mark sat, unfairly elected (5)
I believe the answer is:
stain
'mark' is the definition.
('stain' can be a synonym of 'mark')
'sat unfairly elected' is the wordplay.
'unfairly' indicates anagramming the letters (I've seen this in other clues).
'elected' becomes 'in'.
'sat' anagrammed gives 'sta'.
'sta'+'in'='STAIN'
(Other definitions for stain that I've seen before include "Discolouration" , "Discolour - blemish" , "Discoloured patch" , "Discoloured mark" , "Mark with colour, dirt" .)