Unusual exam covering one alcoholic drink (8)
I believe the answer is:
original
'unusual' is the definition.
(thesaurus)
'exam covering one alcoholic drink' is the wordplay.
'exam' becomes 'oral' (oral exam is a kind of exam).
'covering' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'alcoholic drink' becomes 'gin' (I've seen this before).
'i'+'gin'='igin'
'oral' going around 'igin' is 'ORIGINAL'.
(Other definitions for original that I've seen before include "Primitive, earliest" , "Person of marked individuality" , "Archetype,master copy" , "An oil rig (anag)" , "Pristine; innovative" .)