Beelzebub keeps fish (3)
I believe the answer is:
eel
'fish' is the definition.
(eel is a type of fish)
'beelzebub keeps' is the wordplay.
'keeps' says the answer is hidden in the clue.
'EEL' can be found hidden inside 'beelzebub'.
(Other definitions for eel that I've seen before include "Large elver" , "Apode" , "Proverbially slippery fish" , "Slimy fish" , "Elongated fish" .)