A little drink is wrong for a party leader (3)
I believe the answer is:
sip
'a little drink' is the definition.
(a sip of something is a small drink of it)
'is wrong for a party leader' is the wordplay.
'wrong' indicates anagramming the letters.
'for' means one lot of letters go next to another.
'a party leader' becomes 'p' (leader of the word 'party').
'is' with letters rearranged gives 'si'.
'si'+'p'='SIP'
(Other definitions for sip that I've seen before include "Take small mouthfuls of (drink)" , "Drink daintily" , "Tiny amount of drink" , "Small mouthful of liquid" , "Drink slowly" .)