Monster fish? (5)
I believe the answer is:
troll
I believe this clue is a double definition.
'monster' is the first definition.
(I know that troll is a type of mythical creature)
'fish' is the second definition.
(trolling is a type of fishing)
(Other definitions for troll that I've seen before include "Fairytale monster" , "Fish by trailing a baited line" , "cyber-bully" , "Mythological being" , "Supernatural cave-dweller" .)