Allons donc! = You are joking.

Allumer

Il n’est bois si vert qui ne s’allume” (Clément Marot) = There is nothing so difficult that cannot be done in time.

Alors

Alors comme alors = Wait till that happens, and then we will see what is to be done.

Ambre

Fin comme l’ambre = As sharp as a needle.

[This is said to have originated in the scent of ambergris, which is of a subtle, penetrating nature.]

Amener

Cette preuve est amenée de bien loin = That proof is very far-fetched.