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

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).

One edge case: if your opponent's units have all died and only your newly played Defender unit remains at this battlefield, you (as Defender) win the Combat and Establish Control, which can Conquer if you haven't scored there this turn.