Talk about son, occasionally filthy, showing virtue (8)
I believe the answer is:
chastity
'virtue' is the definition.
('chastity' can be a synonym of 'virtue')
'talk about son occasionally filthy' is the wordplay.
'talk' becomes 'chat' (I've seen this before**).
'about' indicates putting letters inside.
'son' becomes 's'.
'occasionally' indicates alternate letters.
The alternate letters of 'filthy' are 'ity'.
'chat' going around 's' is 'chast'.
'chast'+'ity'='CHASTITY'
'showing' is the link.
(Other definitions for chastity that I've seen before include "Abstinence and purity" , "Pure character" , "Medieval contraceptive device" , "Sexual purity" , "Virtue, celibacy" .)