Not standing for mendacity (5)
I believe the answer is:
lying
I believe this clue is a double definition.
'not standing' is the first definition.
('lie' is the opposite to 'stand')
'mendacity' is the second definition.
(I've seen this before)
'for' acts as a link.
(Other definitions for lying that I've seen before include "Not straight" , "Perjury" , "Saying what is 23" , "Dishonest" , "Invention" .)