Pack of 100 sheep (4)
I believe the answer is:
cram
'pack' is the definition.
(synonyms)
'100 sheep' is the wordplay.
'r00' becomes 'c' (Roman numeral).
'sheep' becomes 'ram' (male sheep).
'c'+'ram'='CRAM'
'of' acts as a link.
(Other definitions for cram that I've seen before include "prepare hastily for the paper" , "Study hard for exam" , "A crush" , "Stuff; study hard" , "Quickly learn - stuff" .)