Unusual exam conceals one trap (8)
I believe the answer is:
original
'unusual' is the definition.
(thesaurus)
'exam conceals one trap' is the wordplay.
'exam' becomes 'oral' (spoken academic test).
'conceals' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'trap' becomes 'gin' (gin trap is a kind of animal trap).
'i'+'gin'='igin'
'oral' placed around 'igin' is 'ORIGINAL'.
(Other definitions for original that I've seen before include "Existing from the start - quite new" , "Phototype" , "Genuine" , "From which a copy is made" , "New and unusual" .)