Thrash young farm animal endlessly (3)
I believe the answer is:
lam
'thrash' is the definition.
(both can mean to hit)
'young farm animal endlessly' is the wordplay.
'young farm animal' becomes 'lamb' (young sheep).
'endlessly' means to remove the last letter.
'lamb' with its final letter removed is 'LAM'.
(Other definitions for lam that I've seen before include "On this, you're in flight, especially from the police" , "hurried flight" , "US criminals commonly escape?" , "Thump; leg it" , "pelt" .)