One fag end’s left unpleasant discharge (5)
I believe the answer is:
ichor
'unpleasant discharge' is the definition.
(archaic term for discharge from a wound)
'one fag end's left' is the wordplay.
'one' becomes 'i' (Roman numeral).
'fag' becomes 'chore' (fag can mean a chore).
'end's left' means to remove the last letter.
'chore' with its final letter taken off is 'chor'.
'i'+'chor'='ICHOR'
(Other definitions for ichor that I've seen before include "divine juice" , "Superior blood" , "Ethereal juice" , ""Blood" of Greek gods" , "flowed through Hades?" .)