Not standing for fibbing (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')
'fibbing' is the second definition.
(I've seen this in another clue)
'for' is the link.
(Other definitions for lying that I've seen before include "Not telling the truth" , "Telling fibs" , "Prone; fibbing" , "Not being disturbed" , "Untruthful - in bed?" .)