Rotten clues mostly deserve to fester? (8)
I believe the answer is:
ulcerate
'fester?' is the definition.
(I've seen this in another clue)
'rotten clues mostly deserve' is the wordplay.
'rotten' is an anagram indicator.
'mostly' means to remove the last letter (most but not all of the word).
'deserve' becomes 'rate' (synonyms).
'clues' with its final letter removed is 'clue'.
'clue' is an anagram of 'ulce'.
'ulce'+'rate'='ULCERATE'
'to' acts as a link.
(Other definitions for ulcerate that I've seen before include "Form open sore" , "Become affected by open sores -- late cure (anag)" , "Erupt into sores" , "Become inflamed and infected" .)