Start to sip convenient drink (6)
I believe the answer is:
shandy
'drink' is the definition.
(shandy is a kind of drink)
'start to sip convenient' is the wordplay.
'start to' says to take the initial letters.
'convenient' becomes 'handy' (synonyms).
The first letter of 'sip' is 's'.
's'+'handy'='SHANDY'
(Other definitions for shandy that I've seen before include "Beer and lemonade mixture" , "Drink made of beer and lemonade" , "Drink of ale and lemonade 96)" , "Lemonade-beer drink" , "Light beer" .)