Started betting on bingo when drunk (6)
I believe the answer is:
blotto
'drunk' is the definition.
(slang term)
'started betting on bingo' is the wordplay.
'started' indicates taking the first letters.
'on' means one lot of letters go next to another.
'bingo' becomes 'lotto' (synonyms).
The initial letter of 'betting' is 'b'.
'b'+'lotto'='BLOTTO'
'when' acts as a link.
(Other definitions for blotto that I've seen before include "wrecked" , "Dead drunk" , "More than happy" , "To bolt away slangily drunk" , "Slangily drunk - ready to bolt" .)