Hardened against (6)
I believe the answer is:
inured
'hardened against' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for inured that I've seen before include "Hardened by familiarity" , "Hardened or calloused, being ruined" , "Hardened as by experience" , "Accustomed to hardship" , "Accustomed to something unpleasant" .)