Astringent compound (4)
(Other definitions for tannin that I've seen before include "Substance in tea, wine, etc" , "It's found in tea and wine" , "Compound used to convert hide into leather" , "Something in red wines" , "It may be found in red wines" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">astringent compound</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 alum that I've seen before include "Name given to a specific chemical compound and a class of them" , "White mineral salt used in medicine and in dyeing" , "double sulphate" , "Compound used in tanning and dying" , "Colourless compound used in dyeing and tanning" .)