Grad (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">grad</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for alum that I've seen before include "Sulphate; maul (anag.)" , "Aluminium/potassium sulphate" , "Colourless compound used in dyeing and tanning" , "Name given to a specific chemical compound and a class of them" , "Potassium compound; IoW sandy bay" .)