Monstrous serpent appears in filthy drain (5)
I believe the answer is:
hydra
'monstrous serpent' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'in filthy drain' is the wordplay.
'in' says the answer is hidden in the clue.
'HYDRA' can be found hidden inside 'filthy drain'.
'appears' acts as a link.
Can you help me to learn more?
(Other definitions for hydra that I've seen before include "Mythological water-monster" , "Mythic monster with nine heads" , "Nine-headed monster killed by Hercules" , "Small red-berried tree" , "Nine-headed monster of Greek myth" .)