First test in which I drink (8)
I believe the answer is:
original
'first' is the definition.
(similar in meaning)
'test in which i drink' is the wordplay.
'test' becomes 'oral' (oral is a type of examination).
'in which' is an insertion indicator.
'drink' becomes 'gin' (gin is a kind of drink).
'i'+'gin'='igin'
'oral' enclosing 'igin' is 'ORIGINAL'.
(Other definitions for original that I've seen before include "Existing from the start - quite new" , "The first, not a copy" , "From which a copy is made" , "An oil rig (anag)" , "Native" .)