Drinker very troublesome to start with (3)
I believe the answer is:
'drinker' is the definition.
(I know that sot is a type of drinker)
'very troublesome to start with' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'to start with' says to take the initial letters.
The initial letter of 'troublesome' is 't'.
(Other definitions for sot that I've seen before include "One's often intoxicated" , "Lush" , "A chronic drinker" , "drunk" , "Habitual drinker" .)