If an opponent uses Reavers to retreat their unit when I move in to contest, do I still have the attacker designation and can I play Cleave to gain Assault stats?

Ruling: Yes, you retain the attacker designation and can play Cleave to benefit from Assault. When Reavers activates, the defender assignment has already occurred, which means you are the attacker.

Sequence:

• Opponent's unit is in a battlefield with Reavers
• You move a unit in to contest
• Attacker (you) and defender (opponent) are assigned
• Opponent activates Reavers to retreat their unit
• Initial chain resolves
• You gain focus and priority as the attacking player
• You can cast Cleave as an action and benefit from Assault

Nuances:

• Cleave is a spell with the action keyword, so it must be played after the initial chain resolves when you have priority
• The attacker/defender assignments happen before Reavers can activate, so retreating doesn't remove your attacker status