Clearcasting will only proc off of the first casting of a spell. As soon as you elect to cast the chance is rolled and then you will enter Clearcast prior to the completion of the spell. If I Clearcast off of the casting of an Arcane Missles then the 15 second countdown on Clearcast begins at the same moment of the 5 second channel for Arcane Missle.
Actually, that's not 100% correct. The Clearcasting is rolled at the moment that the mana is used. For casting bar spells like Frostbolt it will proc at the end of the casting. For channel spells you use the mana at the begining of the spell and that's when the Clearcasting is rolled.
Hopefully that helps.