Brewing agent truly virtuous person (5)
I believe the answer is:
yeast
'brewing agent' is the definition.
(I've seen this in another clue)
'truly virtuous person' is the wordplay.
'truly' becomes 'yea' (I've seen this in another clue).
'virtuous person' becomes 'st' (saint).
'yea'+'st'='YEAST'
(Other definitions for yeast that I've seen before include "fermenter" , "Leavening agent in bread-making" , "Fermenting agent" , "Beer froth agent" , "Fungus used in fermenting" .)