Harmful insect we eradicated (4)
I believe the answer is:
evil
'harmful' is the definition.
(synonyms)
'insect we eradicated' is the wordplay.
'insect' becomes 'weevil' (weevil is a type of beetle).
'eradicated' indicates named letters should be taken away.
'weevil' with 'we' taken away is 'EVIL'.
(Other definitions for evil that I've seen before include "Excellent" , "Morally corrupt" , "Malignant" , "heinous" , "mischief" .)