Live session beginning during leave (5)
I believe the answer is:
exist
'live' is the definition.
('exist' can be a synonym of 'live')
'session beginning during leave' is the wordplay.
'beginning' indicates taking the first letters.
'during' means one lot of letters goes inside another.
'leave' becomes 'exit' ('exit' can be a synonym of 'leave'**).
The initial letter of 'session' is 's'.
's' going into 'exit' is 'EXIST'.
(Other definitions for exist that I've seen before include "Subsist" , "Have objective reality" , "Live" , "Survive" , "Have being or continue living" .)