Thick board (5)
I believe the answer is:
plank
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">thick board</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for plank that I've seen before include "Policy; board" , "Building timber" , "Long piece of wood" , "Length of sawn timber" , "item in political party's programme" .)