Likely (to) (5)
(Other definitions for apt that I've seen before include "Fit, suitable" , "suitable for the circumstances" , "Pertinent" , "Relevant" , "Suitable or likely" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">likely to</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)