First exam includes one trap (8)
I believe the answer is:
original
'first' is the definition.
('original' is associated in meaning with 'first')
'exam includes one trap' is the wordplay.
'exam' becomes 'oral' (oral exam is a kind of exam).
'includes' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'trap' 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 "An oil rig (anag)" , "New and unusual" , "Existing from the start - quite new" , "Not copied or imitated" , "Not derived or copied form something else" .)