Become accustomed (to) (5)
I believe the answer is:
inure
'become accustomed to' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for inure that I've seen before include "Cause to become hardened to" , "Harden or season" , "Accustom (someone) to something unpleasant" , "get tougher" , "Get used to conditions" .)