Small and convenient mixed drink (6)
I believe the answer is:
shandy
'mixed drink' is the definition.
(I've seen this before)
'small and convenient' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' means one lot of letters go next to another.
'convenient' becomes 'handy' (similar in meaning).
's'+'handy'='SHANDY'
(Other definitions for shandy that I've seen before include "Drink of ale and lemonade 96)" , "Light beer" , "Lemonade-beer drink" , "Alcohol" , "Weakly-alcoholic drink" .)