Be initially slow in departure (5)
I believe the answer is:
exist
'be' is the definition.
('exist' can be a synonym of 'be')
'initially slow in departure' is the wordplay.
'initially' says to take the initial letters.
'in' indicates putting letters inside.
'departure' becomes 'exit' ('exit' can be a synonym of 'departure').
The first letter of 'slow' is 's'.
's' put within 'exit' is 'EXIST'.
(Other definitions for exist that I've seen before include "quick!" , "Get a life!" , "Scrape by" , "Have being or continue living" , "Have objective reality" .)