Likely (to) (3)
(Other definitions for apt that I've seen before include "Fit, suitable" , "suitable for the circumstances" , "Pertinent" , "Relevant" , "Suitable or likely" .)
'likely to' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for prone that I've seen before include "Given" , "Lying face downward" , "Prostrate" , "Lying face down or likely to do something" , "Lying face down - inclined" .)
(Other definitions for liable that I've seen before include "Legally answerable" , "To blame" , "Subject to an obligation" , "Responsible at law" , "Apt to" .)