Bet earnings on colour (7)
I believe the answer is:
wagered
'bet' is the definition.
(I've seen this before)
'earnings on colour' is the wordplay.
'earnings' becomes 'wage' (synonyms).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'colour' becomes 'red' (red is a kind of colour).
'wage'+'red'='WAGERED'
(Other definitions for wagered that I've seen before include "Placed a bet" , "Laid down" , "Staked money" , "Gambled" , "Made a bet" .)