If Yasuo - Remorseful (6 Might) moves to an occupied battlefield and his 'When I attack' ability deals 6 damage to a defending unit with 5 Might, does the defender die during cleanup before combat occurs?

Ruling: Yes, the defender will cease to exist during cleanup after Yasuo's ability resolves, so no combat will occur afterwards.

Sequence:

• Yasuo's "When I attack" ability goes on the Initial Chain
• The ability resolves, dealing 6 damage to the defending unit
• Cleanup occurs after the ability resolves
• The defender ceases to exist during cleanup
• No combat occurs because there is no defender