Spoilt Dover sole - cooked too much (8)
I believe the answer is:
overdone
'cooked too much' is the definition.
(I've seen this before)
'spoilt dover sole' is the wordplay.
'spoilt' is an anagram indicator.
'sole' becomes 'one' (synonyms).
'dover' is an anagram of 'overd'.
'overd'+'one'='OVERDONE'
(Other definitions for overdone that I've seen before include "Exaggerated - far from rare" , "Hammed up" , "Cooked too much or carried too far" , "Exaggerated; burnt (meat)" , "Cooked to excess" .)