Protected shell's opening with hardened exterior (7)
I believe the answer is:
insured
'protected shell's' is the definition.
'insured' can be an answer for 'protected' (insuring is a kind of protecting). I'm not sure about the 'shell's' bit.
'opening with hardened exterior' is the wordplay.
'opening with' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'hardened' becomes 'inured' (synonyms).
'exterior' is an insertion indicator (some letters go on the outside of others).
's' inserted within 'inured' is 'INSURED'.
Can you help me to learn more?
(Other definitions for insured that I've seen before include "Protected against loss" , "Covered for loss" , "Covered for burglary?" , "Covered against accident" , "Financially covered" .)