Cud (5)
I believe the answer is:
rumen
'cud' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rumen that I've seen before include "bit of guts (beastly type)" , "Place to digest" , "This organ" , "Cow's first stomach" , "First stomach of a cow - run me (anag)" .)