Established standards in mock exams externally (5)
I believe the answer is:
codes
'established standards' is the definition.
'mock exams externally' is the wordplay.
'mock' becomes 'cod' (codding is a kind of mocking).
'externally' means to remove the middle letters (only the external letters are used).
'exams' with its middle removed is 'es'.
'cod'+'es'='CODES'
'in' acts as a link.
(Other definitions for codes that I've seen before include "Lots of laws" , "Systems for transmitting secret messages" , "Symbolic systems" , "Sets of rules or cryptograms" , "Word systems devised to create secrecy" .)