Monster in filthy drain (5)
I believe the answer is:
hydra
'monster' is the definition.
(I've seen this before)
'in filthy drain' is the wordplay.
'in' indicates a hidden word.
'HYDRA' is hidden within 'filthy drain'.
(Other definitions for hydra that I've seen before include "Greek island" , "Femme fatale of legend" , "Mythological monster with nine heads" , "Stars" , "Many-headed monster of Greek mythology" .)