Ruin betting on fuel (5)
I believe the answer is:
spoil
'ruin' is the definition.
(I know that ruin is a kind of spoil)
'betting on fuel' is the wordplay.
'betting' becomes 'sp'.
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'fuel' becomes 'oil' (oil is a type of fuel).
'sp'+'oil'='SPOIL'
(Other definitions for spoil that I've seen before include "Be too nice to - strip" , "Butcher" , "Deface" , "Having dredged-up material" , "Treat with excessive kindness" .)