Start to live on alcohol (5)
I believe the answer is:
begin
'start' is the definition.
('begin' can be a synonym of 'start')
'live on alcohol' is the wordplay.
'live' becomes 'be' ('be' can be a synonym of 'live').
'on' means one lot of letters go next to another.
'alcohol' becomes 'gin' (gin is a kind of alcohol).
'be'+'gin'='BEGIN'
'to' acts as a link.
(Other definitions for begin that I've seen before include "Israeli prime minister and Nobel Peace Prize winner, d. 1992" , "Partly open" , "Monachem --" , "Commence" , "Israeli statesman" .)