Telling tall tales (5)
I believe the answer is:
lying
'telling tall tales' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lying that I've seen before include "Prone - mendacity" , "Prone; fibbing" , "Flat-out" , "Resting horizontally; fibbing" , "production of pork pies?" .)