i give assault to kennen before moving in. opponent plays void seeker on my 6 might (including 2 assault) kennen. i gust the only unit at the battlefield. does my kennen heal up before it loses assault?>

Yes — the heal happens first; the Assault bonus is still on when the cleanup runs.

The Combat Cleanup invoked in the Resolution Step inserts "3c. Heal all Units" (Rule 466.1.a.1), and Attacker/Defender designations — which are the whole reason Assault is live — aren't removed until combat actually ends (466.7). So the lethal check (323.5) and the heal both evaluate Kennen as a 6-Might attacker: Void Seeker's 4 isn't lethal, then every unit in play is healed to 0 marked damage. Gusting the defender away doesn't cost you the heal either — the Combat Damage Step gets skipped, but 465.3 sends you straight to the Resolution Step and the Combat Cleanup still happens (also FAQ #3522).

One caution: if part of that 6 is a static bonus tied to the unit you Gusted, that's a different story. Keeper of Balance has "+2 [M] while there's a stunned enemy unit here" — if you Gust that stunned unit, Kennen drops to 4 Might instantly, and the kill check at 323.5 (step 3b) runs before the heal at 3c, so he'd die with 4 damage marked. The Assault itself is never the problem; it lasts as long as he holds the Attacker designation.