Sign of illegitimacy as wickedness is evident in deal (3,8)
I believe the answer is:
bar sinister
'sign of illegitimacy' is the definition.
(heraldic term)
'wickedness is evident in deal' is the wordplay.
'wickedness' becomes 'sin' ('sin' can be a synonym of 'wickedness').
'evident in' indicates putting letters inside.
'deal' becomes 'barter'.
'sin'+'is'='sinis'
'sinis' put inside 'barter' is 'BAR-SINISTER'.
'as' is the link.
(Other definitions for bar sinister that I've seen before include "Sign of illegitimacy" , "Indication of illegitimacy" , "heraldic line" , "sign of a bastard" .)