Prone. possibly, to be retailing pork pies? (5)
I believe the answer is:
lying
I believe this is a double definition.
'prone' is the first definition.
(thesaurus)
'to be retailing pork pies?' is the second definition.
(I've seen this before)
'possibly' acts as a link.
(Other definitions for lying that I've seen before include "Prevaricating" , "So one can't stand" , "Deceit" , "Not being disturbed" , "Prone - telling porkies" .)