Covered small hardened cases (7)
I believe the answer is:
insured
'covered' is the definition.
('insure' can be a synonym of 'cover')
'small hardened cases' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'hardened' becomes 'inured' ('inure' can be a synonym of 'harden').
'cases' indicates putting letters inside.
's' placed into 'inured' is 'INSURED'.
(Other definitions for insured that I've seen before include "Covered against mishaps" , "Protected against loss" , "Covered for burglary?" , "Covered against accident" , "Covered, maybe against damage to 23 ..." .)