Whitening chemical (6)
I believe the answer is:
bleach
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">whitening chemical</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 bleach that I've seen before include "Purge" , "Go white" , "Blanche" , "Whiten - disinfectant" , "Whiten with a belch" .)