Cancel posh party after first night (4)
I believe the answer is:
undo
'cancel' is the definition.
(I've seen this before)
'posh party after first night' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'party' becomes 'do' (do can informally mean a party).
'after' is a charade indicator (letters next to each other).
'first night' becomes 'n' (1st letter of 'night').
'do' put after 'n' is 'ndo'.
'u'+'ndo'='UNDO'
(Other definitions for undo that I've seen before include "Loosen; annul" , "Ruin or loose" , "Cause to become loose" , "Overturn" , "Ruin or undermine" .)