A thousand, I wager, is an error (7)
I believe the answer is:
mistake
'an error' is the definition.
('mistake' can be a synonym of 'error')
'a thousand i wager' is the wordplay.
'a thousand' becomes 'M' (Roman numeral for a thousand).
'wager' becomes 'stake' (staking is a kind of wagering).
'm'+'i'+'stake'='MISTAKE'
'is' acts as a link.
(Other definitions for mistake that I've seen before include "Oversight" , "16 down" , "Wrong action or judgment" , "Error" , "Slip" .)