Leave plant out (3,3)
I believe the answer is:
set off
'leave' is the definition.
(set off can mean to depart)
'plant out' is the wordplay.
'plant' becomes 'set' (both can mean to position or place something).
'out' becomes 'off' (I've seen this before).
'set'+'off'='SET-OFF'
(Other definitions for set off that I've seen before include "Depart" , "Display - mount - start journey" , "Begin journey or cause to ignite" , "'Leave on a journey (3,3)'" , "Explode; start journey" .)