One exam covering 2,000 -- that's wrong! (7)
I believe the answer is:
immoral
'wrong' is the definition.
('immoral' can be similar in meaning to 'wrong')
'one exam covering 2000' is the wordplay.
'one' becomes 'i' (Roman numeral).
'exam' becomes 'oral' (oral exam is a kind of exam).
'covering' is an insertion indicator.
'2000' becomes 'mm'.
'i'+'oral'='ioral'
'ioral' placed around 'mm' is 'IMMORAL'.
'that's' acts as a link.
(Other definitions for immoral that I've seen before include "Contrary to religious teaching or natural law" , "Unprincipled or debauched" , "Without a sense of right and wrong" , "Promiscuous" , "Evil" .)