Snake or pig with rook making escape (3)

I believe the answer is:

boa

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'snake' is the definition.
(I know that boa is a type of constrictor)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for boa that I've seen before include "One with scaly body" , "strangler" , "Potential killer" , "Snake; feather stole" , "Wrap" .)

I've seen this clue in The Sun.

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