Cow's mouthful (3)
I believe the answer is:
cud
'cow'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 "Ruminants chew it (3)" , "result of some rumination" , "Re-chewed food of a ruminant" , "Ruminant's regurgitated food" , "it's still in the cow" .)