Brewer's agent's truly virtuous man (5)
I believe the answer is:
yeast
'brewer's agent's' is the definition.
(chemical agent used when brewing alcohol)
'truly virtuous man' is the wordplay.
'truly' becomes 'yea' (I've seen this in another clue).
'virtuous man' becomes 'st' (I've seen this in other clues).
'yea'+'st'='YEAST'
(Other definitions for yeast that I've seen before include "Androgynous" , "Organism causing fermentation" , "Baker's raising agent" , "A raising agent in baking" , "Beer froth agent" .)