Error discovered by end of term, I wager (7)
I believe the answer is:
mistake
'error' is the definition.
(I know that error can be written as mistake)
'end of term i wager' is the wordplay.
'end of' indicates one should take the final letters.
'wager' becomes 'stake' (wager in poker).
The final letter of 'term' is 'm'.
'm'+'i'+'stake'='MISTAKE'
'discovered by' acts as a link.
(Other definitions for mistake that I've seen before include "Slip" , "Error" , "Oversight" , "Wrong action or judgment" , "16 down" .)