Je n’y vois pas clair = I cannot see, it is too dark.
Clair comme le jour (or, comme le soleil en plein midi) = As plain as a pikestaff; As clear as noonday.
Classe
À la rentrée des classes = When school reopens.
Clef
Mettre la clef sous la porte = To run away from one’s creditors; “To bolt.”
La clef dont on se sert est toujours claire = One does not get rusty in what one does every day.
Clerc
Un pas de clerc = A blunder; A false step.
Il ne faut pas parler latin devant les clercs = Do not correct a specialist on his subject.