Dragon Magazine 399, under "Character Themes", introduces a power named "Guardian's Counter." This power is described as follows:
You and the ally shift up to 2 squares as a free action, swapping positions. You become the target of the triggering attack, in place of the ally. After the attack is resolved, you can make a basic attack against the attacker.
It is triggered by:
An ally within 2 squares of you is attacked and you are not included in the attack
If the allies are in a flanking position around a mob like so:
F * * * M * * * C
where F = fighter, M = mob, and C = cleric, can the cleric use this power to take damage in place of the fighter?
Edit: The mob being flanked did not initiate the attack. Another mob initiated the attack some squares away as a ranged attack.