A worker present ultimately during boozy sesh (9)
I believe the answer is:
bartender
'sesh' is the definition.
I can't tell whether this defines the answer.
'a worker present ultimately during boozy' is the wordplay.
'ultimately' indicates one should take the final letters.
'during' is an insertion indicator.
'boozy' becomes 'bender' (I can't explain this - if you can you should give a lot more credence to this answer).
The final letters of 'worker present' are 'rt'.
'a'+'rt'='art'
'art' placed into 'bender' is 'BARTENDER'.
Can you help me to learn more?
(Other definitions for bartender that I've seen before include "Someone serving drinks" , "One serving" , "server of drinks?" , "source of 9 [ROUND] ?" , "One handling counter measures" .)