He's probably drunk, thus about finished (3)
I believe the answer is:
sot
'he's probably drunk' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'thus about finished' is the wordplay.
'thus' becomes 'so' (synonyms).
'about finished' becomes 't' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'so'+'t'='SOT'
Can you help me to learn more?
(Other definitions for sot that I've seen before include "One drinks" , "Boozer" , "Lush" , "A chronic drinker" , "Habitual drunkard" .)