Harden (to) (5)
I believe the answer is:
inure
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">harden to</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for inure that I've seen before include "Cause to become hardened to" , "To give experience" , "Get used to conditions" , "Accustom (someone) to something unpleasant" , "Harden, habituate" .)