Party hard, routinely getting plastered, drinking without getting upset (3,4,4,4)

I believe the answer is:

let your hair down

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'routinely getting plastered drinking without getting upset' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'party hard' is the wordplay.
I cannot quite see how this works, but
'party' could be 'do' (do can informally mean a party) and 'do' is present in the answer.
'hard' could be 'h' (abbreviation used in pencil classifications) and 'h' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Independent.

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