Flat out exaggerating! (5)
I believe the answer is:
lying
This clue could be a double definition.
'flat' is the first definition.
(I've seen this before)
'exaggerating' is the second definition.
The answer and definition can be both to do with communicating as well as being present participle verbs.
Perhaps there's a link between them I don't understand?
'out' is the link.
Can you help me to learn more?
(Other definitions for lying that I've seen before include "So one can't stand" , "Telling untruths" , "Flat-out" , "Prone - mendacity" , "possibly, to be mendacious" .)