Prone to being deceitful (5)
I believe the answer is:
lying
I believe this clue is a double definition.
'prone' is the first definition.
(I've seen this in another clue)
'deceitful' is the second definition.
(I've seen this before)
'to being' is the link.
(Other definitions for lying that I've seen before include "Untruthful - in bed?" , "Not standing" , "Prostrate -- prone" , "Deceitful" , "production of pork pies?" .)