Nudge sheep's behind with stick (4)
I believe the answer is:
prod
'nudge' is the definition.
(synonyms)
'sheep's behind with stick' is the wordplay.
'behind' indicates one should take the final letters.
'with' means one lot of letters go next to another.
'stick' becomes 'rod' (I've seen this before).
The last letter of 'sheep' is 'p'.
'p'+'rod'='PROD'
(Other definitions for prod that I've seen before include "Poke; reminder" , "Poke or spur" , "Push with pointed instrument" , "Poke, stimulate into action" , "Poke (with a finger, etc)" .)