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 tell whether it can be defined by this definition.

'party hard' is the wordplay.
I cannot really understand how this works, but
'party' could be 'do' (do can informally mean a party) and 'do' is found within the answer.
'hard' could be 'h' (abbreviation used in pencil classifications) and 'h' is found within the answer.
This explanation may well be incorrect...

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I've seen this clue in The Independent.

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