Small convenient drink (6)
I believe the answer is:
shandy
'drink' is the definition.
(shandy is a kind of drink)
'small convenient' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'convenient' becomes 'handy' ('handy' can be similar in meaning to 'convenient').
's'+'handy'='SHANDY'
(Other definitions for shandy that I've seen before include "Beer/lemonade mixture" , "Beer-lemonade mix" , "Weakly-alcoholic drink" , "Beer and lemonade mixture" , "Alcohol" .)