Check waste matter's hidden (4)
I believe the answer is:
stem
'check' is the definition.
(thesaurus)
'waste matter's hidden' is the wordplay.
'hidden' says the answer is hidden in the clue.
'STEM' is hidden within 'waste matter'.
(Other definitions for stem that I've seen before include "Vertical part of flower or drinking glass" , "root" , "Hold back - stock - check" , "race" , "Stalk - check" .)