So an initial teetotaller becomes a drunkard! (3)
I believe the answer is:
sot
'a drunkard' is the definition.
('sot' can be a synonym of 'drunkard')
'so an initial teetotaller' is the wordplay.
'an initial' says to take the initial letters.
The initial letter of 'teetotaller' is 't'.
'so'+'t'='SOT'
'becomes' is the link.
(Other definitions for sot that I've seen before include "Toper" , "Habitual drunkard" , "One drinks" , "Habitual drinker" , "drunk" .)