I have Clocktower Guardian defend a bf, he dies and I play a Unit for free now. Can I play him to the bf since the deathknell trigger is before conquer?

Yes — you can play the banished card to the battlefield Clocktower Guardian died at.

The Deathknell is put on the chain during the kill (Rule 383.2.c.3), so you still control that battlefield when it resolves: control can't change while a Combat is ongoing (190.4.b), and a cleanup can't take a battlefield from you while there's a pending chain item (190.4.c / 323.5). The Deathknell then resolves during the Combat Cleanup, which is before Determine Combat Result and the Establish Control/Conquer step (Rule 468), so that battlefield is still a legal location for you to play to. This is the same defender case as FAQ #2086 — it's the attacker in that position who can't play to the battlefield they're attacking.

Two corrections worth noting: the ability only ignores the card's Energy cost, so you still pay any Power it has. And the unit you play arrives during the cleanup, so it doesn't deal damage this combat — it's just present when the result is checked, which means both players have units there and the combat comes back as No Result, re-staging a Showdown and Combat rather than letting the attacker conquer on that pass.