The insecure should, when they lose heart (6)
I believe the answer is:
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for insure that I've seen before include "Rue sin (anag)" , "Protect (something) against loss" , "Protect by taking out a policy" , "Protect against possible loss" , "do business at Lloyds?" .)