Eeyore's upset by a blemish (7)
I believe the answer is:
eyesore
'a blemish' is the definition.
(eyesore is a kind of blemish)
'eeyore's upset' is the wordplay.
'upset' indicates an anagram.
'eeyores' is an anagram of 'EYESORE'.
'by' is the link.
(Other definitions for eyesore that I've seen before include "Ugly and offensive sight" , "Blemish" , "Monstrosity" , "Blot on the landscape" , "Something exceedingly ugly" .)