Harden cover son's left out (5)
I believe the answer is:
inure
'harden' is the definition.
(I know that harden can be written as inure)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for inure that I've seen before include "get tougher" , "Harden against, make tough" , "Accustom (someone) to something unpleasant" , "Come to accept" , "Steel" .)