Not standing for such mendacity? (5)
I believe the answer is:
lying
I believe this is a double definition.
'not standing' is the first definition.
('lie' is the opposite to 'stand')
'mendacity?' is the second definition.
(thesaurus)
'for such' is the link.
(Other definitions for lying that I've seen before include "Telling falsehoods" , "Perjury" , "Prone - mendacity" , "Prone; fibbing" , "Deviating from truth" .)