First Labour leader picked up mixed drink (3)
I believe the answer is:
kir
'mixed drink' is the definition.
(I know that Kir is a IBA official cocktail)
'first labour leader picked up' is the wordplay.
'first labour leader' becomes 'Keir' (Keir Hardie, founder of the Labour Party).
'picked up' shows a homophone (sound like).
'keir' is a homophone of 'KIR'.
(Other definitions for kir that I've seen before include "White wine with blackcurrant syrup" , "Drink of white wine and cassis" , "Cassis-flavoured wine" , "Blackcurrant drink" , "One that's drunk" .)