Mendacious (5)
I believe the answer is:
lying
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mendacious</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for lying that I've seen before include "not to be trusted?" , "Saying what is 23" , "Telling fibs" , "Not telling the truth" , "Prone; fibbing" .)