Telling untruths flat out? (5)

I believe the answer is:

lying

I'm a little stuck... Click here to teach me more about this clue!

'telling untruths' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for lying that I've seen before include "not to be trusted?" , "Prone - telling porkies" , "Saying what is 23" , "Prostrate -- prone" , "production of pork pies?" .)

I've seen this clue in The Sun.

Want a hint initially instead of a full solution? Install my app