After vacation, slowly check plan of action (6)
I believe the answer is:
system
'plan of action' is the definition.
(system is a kind of plan of action)
'after vacation slowly check' is the wordplay.
'after vacation' suggests removing the centre (the inside letters must vacate).
'check' becomes 'stem' (I've seen this before).
'slowly' with its middle taken out is 'sy'.
'sy'+'stem'='SYSTEM'
(Other definitions for system that I've seen before include "Technique, method" , "Method of organisation" , "Set of things working together" , "Set of interrelated parts working together" , "The government" .)