Beelzebub was back (5)
I believe the answer is:
devil
'beelzebub' is the definition.
(synonyms)
'was back' is the wordplay.
'was' becomes 'lived' (to live is to be).
'back' says the letters should be written backwards.
'lived' reversed gives 'DEVIL'.
(Other definitions for devil that I've seen before include "Terror" , "Evil supernatural being" , "Lucifer" , "Demon" , "Wicked person" .)