Bet it's a long time between start and end of war (5)
I believe the answer is:
wager
'bet' is the definition.
('wager' can be a synonym of 'bet')
'a long time between start and end of war' is the wordplay.
'a long time' becomes 'age' ('age' can be a synonym of 'long time').
'between' is an insertion indicator.
'start and end of' means to remove the middle letters (only the first and last letter remain).
'war' with its middle removed is 'wr'.
'age' placed into 'wr' is 'WAGER'.
'it's' is the link.
(Other definitions for wager that I've seen before include "Put on a bit" , "stake required" , "Bet, gamble" , "One making pledge" , "Punt" .)