Accustom (5)
I believe the answer is:
inure
'accustom' is the definition.
(inuring is a kind of accustoming)
This is the entire clue.
(Other definitions for inure that I've seen before include "Harden or season" , "Cause to become hardened to" , "Temper, toughen" , "Steel" , "Come to accept" .)