The defend move in dungeon world allows you to spend hold to
Halve the attack’s effect or damage
However it is not clear to me whether this occurs before or after you reduce the damage by armor. The description for armor in the SRD doesn't clarify either.
It protects you from harm and absorbs damage. When you take damage, subtract your armor from the total.
My suspicion is that the damage is halved first, because it is done "when you or the thing you defend is attacked" while the armor reduction is done "When you take damage", and it seems like the former precedes the latter, but I can't really find a source to back that up.