Toughen — acclimatise (5)
I believe the answer is:
inure
This clue is a double definition.
'toughen' is the first definition.
(I've seen this in another clue)
'acclimatise' is the second definition.
(I've seen this in another clue)
(Other definitions for inure that I've seen before include "Harden or accustom as to something unpleasant" , "Accustom (someone) to something unpleasant" , "Harden, make accustomed to" , "get tougher" , "Harden, habituate" .)