Olympian takes a little paracetamol after surprise parties (5-3)

'parties' is the definition.
(a booze-up is an alcohol-fuelled party)

'olympian takes a little paracetamol after surprise' is the wordplay.
'olympian' becomes 'zeus' (Greek god who lived on Mount Olympus).
'takes' indicates putting letters inside.
'a little' says to take the initial letters.
'after' says to put letters next to each other.
'surprise' becomes 'boo' ('boo!' is shouted to surprise someone).
The initial letter of 'paracetamol' is 'p'.
'zeus' enclosing 'p' is 'zeups'.
'zeups' after 'boo' is 'BOOZE-UPS'.