Tail a pig is allowed when young (3)

I believe the answer is:

let

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'tail' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for let that I've seen before include "Allow or rent" , "Umpire's decision" , "Allow; leased" , "Permit or rent" , "Allow; tennis fault" .)

I've seen this clue in the Evening Standard.

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