Be first to leave? (5)
I believe the answer is:
start
This clue is a double definition.
'be first' is the first definition.
(I've seen this before)
'leave?' is the second definition.
(starting is a kind of leaving)
'to' acts as a link.
(Other definitions for start that I've seen before include "launch" , "Origin" , "begin an argument?" , "Onset" , "Opening - jump" .)