The setter's backed bet in error (7)
I believe the answer is:
mistake
'error' is the definition.
('mistake' can be a synonym of 'error')
'the setter's backed bet' is the wordplay.
'the setter's' becomes 'im' (I've seen this in other clues).
'backed' shows that the letters should be reversed in order.
'bet' becomes 'stake' (staking is a kind of betting**).
'im' reversed gives 'mi'.
'mi'+'stake'='MISTAKE'
'in' acts as a link.
(Other definitions for mistake that I've seen before include "Oversight" , "Fault" , "Error" , "Slip" , "Wrong action or judgment" .)