Like this bachelor, before party grew serious (7,2)

I believe the answer is:

sobered up

'grew serious' is the definition.
(to sober up is to become serious or sober)

'like this bachelor before party' is the wordplay.
'like this' becomes 'so'.
'bachelor' becomes 'b' (abbreviation e.g. BA=Bachelor of Arts).
'before' becomes 'ere' (poetic term for 'before').
'party' becomes 'dup' (Democratic Unionist Party).
'so'+'b'+'ere'+'dup'='SOBERED UP'

I've seen this clue in The Times.

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