About to depart or part forcefully (6)
I believe the answer is:
cleave
'part forcefully' is the definition.
(to cleave is to split in two)
'about to depart' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'to depart' becomes 'leave' ('leave' can be a synonym of 'depart').
'c'+'leave'='CLEAVE'
'or' acts as a link.
(Other definitions for cleave that I've seen before include "Split or cling to" , "Split with the grain" , "Separate with violence" , "To separate and to adhere" , "Fissure" .)