Pack in Reims getting away from imposters (4)
I believe the answer is:
stop
'pack in' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for stop that I've seen before include "Post (anag.)" , "Hold on" , "Cease, desist, whoa" , "Standstill" , "cut out" .)