It's legal to have relationships in it (3)

I believe the answer is:

law

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'it' is the definition.
The definition suggests a singular noun which matches the answer.

I can't explain the remainder of the clue.

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(Other definitions for law that I've seen before include "Collection of rules for a society" , "Body of rules of conduct for a society" , "Rule prohibiting certain actions" , "Official rule" , "Rule of conduct" .)

I've seen this clue in the Evening Standard.

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