Relents when you allow to have a drink (4,2)

I believe the answer is:

lets up

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'relents when' is the definition.
'lets up' can be an answer for 'relents' (I've seen this in another clue). I am not sure about the 'when' bit.

'allow to have a drink' is the wordplay.
'allow' becomes 'let' (synonyms).
'to have a drink' becomes 'sup' (I've seen this in other clues).
'let'+'sup'='LETS UP'

'you' is the link.

Can you help me to learn more?

(Another definition for lets up that I've seen is " Abates, eases up".)

I've seen this clue in the Evening Standard.

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