Can you attack with a creature that has defender if an effect says it can attack as though it didn't have defender?

Defender is a static ability that prohibits a creature from attacking (CR 702.3a). Normally, a creature with defender cannot be declared as an attacker during the declare attackers step (CR 508.1c).

However, certain card effects can override this restriction by granting permission to attack as though the creature didn't have defender. When such an effect is in place, the restriction imposed by defender is effectively ignored for the purpose of declaring attackers (CR 702.3b).

This works because Magic's rules treat a granted ability to attack 'as though it didn't have defender' as a specific exception that supersedes the general prohibition. The creature still technically has defender, but the overriding effect nullifies its restriction (CR 613, interaction of continuous effects).

Example: You control a Wall of Omens (which has defender) and a Assault Formation is on the battlefield. Assault Formation says creatures you control can attack as though they didn't have defender. You may freely declare Wall of Omens as an attacker that combat phase.

Note that the creature still has defender in every other context — for example, abilities that care about whether a permanent 'has defender' will still see it. Only the attacking restriction is bypassed.