Be prone to untruth (3)
I believe the answer is:
lie
This clue is a double definition.
'be prone' is the first definition.
(I've seen this before)
'untruth' is the second definition.
(I know that lie is a type of untruth)
'to' acts as a link.
(Other definitions for lie that I've seen before include "found" , "Assume recumbent posture" , "Disinformation" , "Falsity" , "Be prostrate" .)