Assume a horizontal position (3)
I believe the answer is:
lie
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">assume a horizontal position</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 lie that I've seen before include "Fib - be prone" , "Pork pie" , "Be prostrate" , "Perversion of the truth" , "Stay hidden" .)