Liable, prone (3)
I believe the answer is:
apt
I believe this is a double definition.
'liable' is the first definition.
(synonyms)
'prone' is the second definition.
(I've seen this before)
(Other definitions for apt that I've seen before include "Proper" , "Appropriate for Pat" , "Fit, suitable" , "Liable" , "Tending" .)