Type of frost (4)
I believe the answer is:
hoar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">type of frost</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for hoar that I've seen before include "Greyish-white with age or frost" , "Type of frost" , "Rime" , "Here are ice crystals" , "A kind of white frost" .)