Toughen — acclimatise (5)
I believe the answer is:
inure
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">toughen</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">acclimatise</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(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" .)