A pair of military leaders turned up for drink (5)
I believe the answer is:
cocoa
'for drink' is the definition.
(cocoa is a kind of drink)
'a pair of military leaders turned up' is the wordplay.
'a pair of military leaders' means two substitutions for 'military leader'.
'military leader' becomes 'co'.
'military leader' becomes 'co'.
'turned up' becomes 'a' (I can't justify this - if you can you should believe this answer much more).
'co'+'co'+'a'='COCOA'
Can you help me to learn more?
(Other definitions for cocoa that I've seen before include "Light brown" , "product of beans!" , "It's made from the seed of the cacao tree" , "Chocolate powder" , "Warming bedtime drink" .)