Is prone to untruth? (5)
I believe the answer is:
lying
I believe this is a double definition.
'is prone' is the first definition.
(I've seen this in another clue)
'untruth?' is the second definition.
(I know that lying is a type of falsity)
'to' acts as a link.
(Other definitions for lying that I've seen before include "retailing pork pies" , "Spread out" , "Mendacious; recumbent" , "Not straight" , "Deceit" .)