Bet salary on first of runners (5)
I believe the answer is:
wager
'bet' is the definition.
('wager' can be a synonym of 'bet')
'salary on first of runners' is the wordplay.
'salary' becomes 'wage' ('wage' can be a synonym of 'salary').
'on' says to put letters next to each other.
'first of' indicates taking the first letters.
The initial letter of 'runners' is 'r'.
'wage'+'r'='WAGER'
(Other definitions for wager that I've seen before include "Punt" , "better to put this on?" , "One making pledge" , "(Place a) stake" , "Risk; stake" .)