Trial drug, snorting one line (5)
I believe the answer is:
pilot
'trial' is the definition.
(synonyms)
'drug snorting one line' is the wordplay.
'drug' becomes 'pot' (slang term for cannabis).
'snorting' means one lot of letters goes inside another (snort can mean to ingest through the nose).
'one' becomes 'i' (Roman numeral).
'line' becomes 'l' (used when specifying particular lines from a poem).
'i'+'l'='il'
'pot' enclosing 'il' is 'PILOT'.
(Other definitions for pilot that I've seen before include "Plane's operator" , "who shows the way?" , "Drive" , "Aircraft captain" , "Navigator" .)