Spill e.g. a toilet's contents in a smooth flow (6)
I believe the answer is:
legato
'a smooth flow' is the definition.
(musical term)
'spill e g a toilet's contents in' is the wordplay.
'in' indicates a hidden word.
'LEGATO' can be found hidden inside 'spill e g a toilets contents'.
(Other definitions for legato that I've seen before include "with smooth execution" , "Smoothly-flowing" , "scored smoothly" , "play on without pause" , "Musically smooth" .)