Prevents any escape of certain animals (5)

I believe the answer is:

seals

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'animals' is the definition.
The answer is an example.

I don't understand the rest of the clue.

(Other definitions for seals that I've seen before include "Fastens or closes securely" , "Makes waterproof" , "They make an impression" , "Animals spotted on coast" , "they're impressive" .)

I've seen this clue in the Evening Standard.

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