Prone to be untruthful (5)
I believe the answer is:
lying
I believe this clue is a double definition.
'prone' is the first definition.
(thesaurus)
'untruthful' is the second definition.
('lying' can be similar in meaning to 'untruthful')
'to be' acts as a link.
(Other definitions for lying that I've seen before include "Spread out" , "Resting horizontally; fibbing" , "Anything but upright" , "telling stories" , "Saying what is 23" .)