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