Ruminant's mouthful (3)
I believe the answer is:
cud
'ruminant's mouthful' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cud that I've seen before include "Something to be chewed" , "It's chewed by a ruminant" , "Re-chewed food of a ruminant" , "result of some rumination" , "THIRD PART OF 23" .)