Drink that's finally convenient (6)
I believe the answer is:
shandy
'drink' is the definition.
(shandy is a kind of drink)
'that's finally convenient' is the wordplay.
'finally' indicates one should take the final letters.
'convenient' becomes 'handy' ('handy' can be similar in meaning to 'convenient').
The last letter of 'thats' is 's'.
's'+'handy'='SHANDY'
(Other definitions for shandy that I've seen before include "mixed drink" , "A misture of ale and lemonade" , "Beer/lemonade drink" , "Alcohol" , "Drink of ale and lemonade 96)" .)