in bf1 attack hahs 1 unit with 5 might. Defence has a Shadow Clone and a Zed, Without a sound. What hahppens in the end of showdown?

The attacker loses: their 5-Might unit is killed and the defender keeps control of bf1.

In the Combat Damage Step (Rule 465), the defender's pool is Zed (5 Might) + Shadow Clone (0 Might) = 5 damage, which is lethal to the attacker's unit — it dies. The attacker's 5-damage pool can only put lethal on one defender, and the attacker picks which one:

• All 5 on Zed → Zed dies, the 0-Might clone survives and the defender holds the battlefield.
• 1 on the clone, 4 on Zed → the clone dies (any non-zero damage is lethal to a 0-Might unit), Zed survives and heals back to full at Combat Cleanup.

Either way the defender is the only player with units left, so they win the combat and establish/retain control (Rule 466.3, 466.5). Zed's Assault and the clone's "When I attack" trigger don't apply — neither is an attacker (Rule 464.2.c).