Water monster hiding in filthy drains (5)
I believe the answer is:
hydra
'water monster' is the definition.
(I've seen this before)
'hiding in filthy drains' is the wordplay.
'hiding' says the answer is hidden in the clue.
'HYDRA' can be found hidden inside 'in filthy drains'.
(Other definitions for hydra that I've seen before include "that regenerated after cuts at the top" , "Proliferating-head monster" , "Many-headed foe of Hercules" , "Monster Hercules slew" , "Multi-headed monster killed by Hercules" .)