Lock up penguin killer (4)

I believe the answer is:

seal

I believe this is a double definition.

'lock up' is the first definition.
(thesaurus)

'penguin killer' is the second definition.
(seals hunt penguins)

(Other definitions for seal that I've seen before include "Stamp of approval" , "Mammal seen swimming" , "Stick down" , "Authorise - plug" , "Member of an elite force within the US Navy" .)

I've seen this clue in The Times.

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