Yes, it's a cleft sentence. But if you are asking about clauses:
if you have to finite verbs, you'll have a clause for each finite verb:
when you start to be afraid of death
that you learn to appreciate life
So, it's two clauses complementing each other, and giving the sentence full meaning. If you want to give them names, you can call the first a wh- clause, and the second a that-clause.
Each clause begins with a "complementizer" (when, that), a word that turns an independent clause (you start to be afraid of life/you learn to appreciate life) into a subordinate clause (when you start to be afraid of life/that you learn to appreciate life).