Become more viscous (7)
I believe the answer is:
thicken
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">become more viscous</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 thicken that I've seen before include "Become more viscous" , "get fatter?" , "(Of a plot) become more complicated" , "Make more viscous" , "Clot" .)