Start packing up before the end (4,2)
I believe the answer is:
kick in
'start' is the definition.
(I know that kick in is a more specific form of the action start)
'packing up before the end' is the wordplay.
'packing up' becomes 'kicking' (kick can mean to pack up or quit).
'before the end' means to remove the last letter (all letters before the one at the end).
'kicking' with its last letter taken off is 'KICK IN'.
(Other definitions for kick in that I've seen before include "get going" , "Start working" , "Come into effect" , "Start to take effect" , "work" .)