Wrong to accept thanks for blemish (5)
I believe the answer is:
stain
'for blemish' is the definition.
(stain is a kind of blemish)
'wrong to accept thanks' is the wordplay.
'wrong' becomes 'sin' (wrong can mean a sin or crime).
'to accept' indicates putting letters inside.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'sin' placed around 'ta' is 'STAIN'.
(Other definitions for stain that I've seen before include "Black mark for saint" , "Discoloration" , "Colour, dye" , "Tarnish" , "Blotch on satin" .)