Be a sheep, yet attack! (4,2)

I believe the answer is:

beat up

I'm a little stuck... Click here to teach me more about this clue!

'be a sheep yet attack' is the definition.
I can't tell whether this definition defines the answer.

'be a sheep' is the wordplay.
'sheep' becomes 'tup' (tup is a kind of sheep).
'be'+'a'+'tup'='BEAT-UP'

Can you help me to learn more?

(Other definitions for beat up that I've seen before include "Dilapidated" , "Assault violently, batter" , "Assault violently and injure" , "Slang for assault violently, belabour" , "mug" .)

I've seen this clue in the Evening Standard.

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