Mark -good man - one good with mum (6)
I believe the answer is:
stigma
'mark' is the definition.
('stigma' can be a synonym of 'mark')
'good man one good with mum' is the wordplay.
'good man' becomes 'st'.
'one' becomes 'i' (Roman numeral).
'good' becomes 'g' (abbreviation).
'with' means one lot of letters go next to another.
'mum' becomes 'ma' ('ma' can be a synonym of 'mum').
'st'+'i'+'g'+'ma'='STIGMA'
(Other definitions for stigma that I've seen before include "Mark of dishonour" , "Disgraceful sign" , "Sign of disgrace" , "Mark of infamy or disgrace" , "Disgraceful mark" .)