Like hens, eat a quantity of grain (4)
I believe the answer is:
peck
This clue is a double definition.
'like hens eat' is the first definition.
(hens peck at their food)
'a quantity of grain' is the second definition.
(peck can mean an amount of grain)
(Other definitions for peck that I've seen before include "Perfunctory kiss" , "One quarter of a bushel" , "quantity of dry stuff" , "Attack with beak" , "lot of pints" .)