Fiend existed the wrong way (5)
I believe the answer is:
devil
'fiend' is the definition.
(synonyms)
'existed the wrong way' is the wordplay.
'existed' becomes 'lived' ('live' can be a synonym of 'exist').
'the wrong way' shows that the letters should be reversed in order.
'lived' written backwards gives 'DEVIL'.
(Other definitions for devil that I've seen before include "Coat spicily; do hackwork" , "Beelzebub, Satan" , "Archfiend" , "Imp" , "Powerful evil spirit" .)