Synthetic fibre or paint (7)
I believe the answer is:
acrylic
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">synthetic fibre</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">paint</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(acrylic paint is a kind of paint)</span><br/ ><br/ ></span>
(Other definitions for acrylic that I've seen before include "Type of paint or textile fibre" , "Type of hard paint" , "Type of twenty Dn" , "Pungent acid" , "Textile fibre - icy Carl (anag)" .)