Painting on three panels (8)
I believe the answer is:
triptych
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">painting on three panels</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 triptych that I've seen before include "Altarpiece" , "Painting on three hinged panels" , "Three-panelled picture" , "panel" , "Three-part painting" .)