Terribly evil and unpleasant (4)
I believe the answer is:
vile
'unpleasant' is the definition.
(synonyms)
'terribly evil' is the wordplay.
'terribly' indicates an anagram.
'evil' is an anagram of 'VILE'.
'and' acts as a link.
(Other definitions for vile that I've seen before include "Live in degenerate way" , "Terrible" , "Extremely unpleasant" , "Despicable in evil way" , "Loathsome - draw veil over it" .)