If i use glasc mixologist to play units at a battlefield i was defending after combat damage, do i now become an attacker or do i stay a defender on that battlefield

Incorrect answer

You stay the Defender. Attacker/Defender is locked in at the start of the Combat Showdown: the Attacker is the player whose unit applied Contested, and everyone else is the Defender (Rules 464.2.c.1–.2). Playing a new unit via Glasc Mixologist's Deathknell doesn't change that — a unit that arrives at the battlefield after designations are set simply inherits its controller's existing designation during the next Cleanup (Rule 323.2.a). So your restored unit enters as a Defender and counts as a defending unit in the damage step/sums. Worth noting the flip side: because the Defender is the one who already controls the battlefield, this play is legal for you here — but if Glasc had died while you were the Attacker, you couldn't play the unit to that battlefield at all (you don't control it until you conquer).