Make thin or thinner (9)
I believe the answer is:
attenuate
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">make thin or thinner</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for attenuate that I've seen before include "Reduce (a signal)" , "Make thin or thinner" , "Slender" , "Reduce in strength" , "Dilute" .)