Rest have told untruths before (3,4)
I believe the answer is:
lie down
'rest' is the definition.
(I've seen this before)
'have told untruths before' is the wordplay.
'have' becomes 'own' ('own' can be a synonym of 'have').
'told untruths' becomes 'lied' (I've seen this before).
'before' is a charade indicator (letters next to each other).
'own' after 'lied' is 'LIE-DOWN'.
(Other definitions for lie down that I've seen before include "Rest (with or without sleep)" , "Become 14" , "Take rest; be submissive (under)" , "Put your feet up" , "Take to one's bed" .)