Million I bet in error (7)
I believe the answer is:
mistake
'error' is the definition.
('mistake' can be a synonym of 'error')
'million i bet' is the wordplay.
'million' becomes 'm' (abbreviation).
'bet' becomes 'stake' (staking is a kind of betting).
'm'+'i'+'stake'='MISTAKE'
'in' acts as a link.
(Other definitions for mistake that I've seen before include "Fault" , "Error" , "16 down" , "Wrong action or judgment" , "Oversight" .)