Blemish isn't a problem (5)
I believe the answer is:
stain
'blemish' is the definition.
(stain is a kind of blemish)
'isn't a problem' is the wordplay.
'problem' indicates anagramming the letters.
'isnt'+'a'='isnta'
'isnta' with letters rearranged gives 'STAIN'.
(Other definitions for stain that I've seen before include "Mark with colour, dirt" , "Blotch on satin" , "Spot - shame" , "Discolouring mark" , "Unsightly mark" .)