Water underground comes from toilet here (5)
I believe the answer is:
lethe
'water underground' is the definition.
(mythical river of the underworld)
'from toilet here' is the wordplay.
'from' says the answer is hidden in the clue.
'LETHE' can be found hidden inside 'toilet here'.
'comes' acts as a link.
(Other definitions for lethe that I've seen before include "where elephants never drink?" , "Oblivion; river of the underworld" , "Mythological river in Hades" , "What could make one forget" , "River whose waters caused the souls of the dead to forget completely their life on Earth" .)