A worker present ultimately during boozy sesh (9)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

A (from clue text) + last letters (ultimately) of [worke] R [presen] T , inserted into BENDER (boozy sesh = a long drinking session). Clue-as-definition: a bender typically takes place in a bar, or often multiple bars one after another, staffed by bartenders.