Rats on wager and then beams (7)
I believe the answer is:
betrays
'rats on' is the definition.
(I've seen this in another clue)
'wager and then beams' is the wordplay.
'wager' becomes 'bet' (synonyms).
'and then' is a charade indicator (letters next to each other) (I've seen this in other clues).
'beams' becomes 'rays' (rays of light).
'bet'+'rays'='BETRAYS'
(Other definitions for betrays that I've seen before include "Gives away" , "Double-crosses" , "Shows unintentionally, say feelings" , "Sells" , "shops" .)