Stuff many a sheep (4)
I believe the answer is:
cram
'stuff' is the definition.
(I know that cram is a type of stuff)
'many a sheep' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for r00).
'a sheep' becomes 'ram' (male sheep).
'c'+'ram'='CRAM'
(Other definitions for cram that I've seen before include "Quickly learn - stuff" , "Crowd or pack to capacity" , "A crush" , "Stuff; study hard" , "Feed greedily" .)