Prevent most of the skin blemish (6)
I believe the answer is:
thwart
'prevent' is the definition.
(thwarting is a kind of preventing)
'most of the skin blemish' is the wordplay.
'most of' means to remove the last letter.
'skin blemish' becomes 'wart' (I've seen this before).
'the' with its last letter taken off is 'th'.
'th'+'wart'='THWART'
(Other definitions for thwart that I've seen before include "Prevent from accomplishing something" , "Prevent from succeeding" , "Frustrate or foil" , "Frustrate and obstruct" , "seat for oarsman" .)