Virtuous person knowing formal behaviour (6)
I believe the answer is:
starch
'formal behaviour' is the definition.
(starch can mean stiffness or formal behaviour)
'virtuous person knowing' is the wordplay.
'virtuous person' becomes 'st' (saint).
'knowing' becomes 'arch' (I've seen this before).
'st'+'arch'='STARCH'
(Other definitions for starch that I've seen before include "Stuff that stiffens, say collars" , "Charts (anag) - formality" , "Stiffening preparation used in laundering" , "Stiffness; carbohydrate" , "Reserve" .)