Feedbag fill (4)
I believe the answer is:
oats
'feedbag fill' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oats that I've seen before include "A cereal crop" , "Grain for porridge" , "Sow wild ones in disreputable youth" , "Seed grasses" , "Porridge ingredients" .)