After work, one Greek character returned narcotic substance (5)
I believe the answer is:
opium
'narcotic substance' is the definition.
(I know that opium is narcotic)
'after work one greek character returned' is the wordplay.
'after' is a charade indicator (letters next to each other).
'work' becomes 'op' (operation, work done in an operating theatre).
'one' becomes 'i' (Roman numeral).
'greek character' becomes 'mu' (letter from the Greek alphabet).
'returned' shows that the letters should be reversed in order.
'mu' backwards is 'um'.
'op' next to 'i' is 'opi'.
'opi'+'um'='OPIUM'
(Other definitions for opium that I've seen before include "Narcotic drug" , "powerful drug" , "Poppy-based drug" , "Addictive drug made from poppies" , "Poppy drug" .)