Prone to deceive (5)
I believe the answer is:
lying
'prone' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for lying that I've seen before include "Anything but upright" , "Prone (to fabrications?)" , "Prone - telling porkies" , "Not being disturbed" , "dishonesty" .)