Leave caveat in motion (6)
I believe the answer is:
vacate
'leave' is the definition.
(vacating is a kind of leaving)
'caveat in motion' is the wordplay.
'in motion' is an anagram indicator (I've seen this in other clues).
'caveat' anagrammed gives 'VACATE'.
(Other definitions for vacate that I've seen before include "Quit" , "Caveat - cease to occupy" , "Withdraw as from premises" , "Clear out and leave empty" , "Leave behind empty" .)