Contributor to second row that's in a canal (4)

I believe the answer is:

lock

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'canal' is the definition.
Although both the answer and definition are verbs in their base form, I can't understand how they can define each other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for lock that I've seen before include "Stop moving" , "Security device - bit of hair" , "Mechanism to keep closed, door say" , "part of 10 [CANAL]" , "Rugby player" .)

I've seen this clue in The Guardian.

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