Start to exist on booze (5)
I believe the answer is:
begin
'start' is the definition.
(I know that start can be written as begin)
'exist on booze' is the wordplay.
'exist' becomes 'be' (synonyms).
'on' says to put letters next to each other.
'booze' becomes 'gin' (gin is a kind of booze).
'be'+'gin'='BEGIN'
'to' acts as a link.
(Other definitions for begin that I've seen before include "Commence" , "Make a start" , "former Israeli Nobel peace prize winner" , "Israeli prime minister and Nobel Peace Prize winner, d. 1992" , "Israeli statesman" .)