Start to live on drink (5)
I believe the answer is:
begin
'start' is the definition.
(I know that start is a more specific form of begin)
'live on drink' is the wordplay.
'live' becomes 'be' (synonyms).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'drink' becomes 'gin' (gin is an alcoholic drink).
'be'+'gin'='BEGIN'
'to' is the link.
(Other definitions for begin that I've seen before include "Commence" , "Israeli prime minister who shared the Nobel Peace Prize with President Sadat" , "Make a start" , "Clandestine, furtive" , "Launch" .)