Convert into bone (6)
I believe the answer is:
ossify
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">convert into bone</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ossify that I've seen before include "become hardened" , "Change into bone, or become unprogressive" , "Make rigid" , "Solidify; turn into bone" , "Become hard and bony with age" .)