What lie does Lochinvar tell so that father allows him to dance with Ellen?

Lochinvar doesn't actually tell a lie to Ellen's father, but rather uses a clever evasion.

Here's what happens:

* Ellen's father, the Laird of Buccleuch, is hesitant to let Lochinvar dance with Ellen because he knows the young man is from an enemy clan.

* Lochinvar asks for "one little minute" to dance with Ellen, claiming it's a "custom of old" and a "courtesy to the fair" (meaning it's an established social norm).

* The Laird, wanting to uphold tradition and appear polite, reluctantly allows the dance.

Lochinvar doesn't outright lie, but he does use a clever ploy to get what he wants. He plays on the Laird's desire to uphold tradition and avoid appearing rude. It's more of a "gentle deception" than a full-fledged lie.