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' suggests the final letters.
'during' indicates putting letters inside.
'boozy' becomes 'bender' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The final letters of 'worker present' are 'rt'.
'a'+'rt'='art'
'art' inserted 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" .)