Malign black hearts concealing terrible crimes (8)
I believe the answer is:
besmirch
'malign' is the definition.
(I know that malign is a more specific form of the action besmirch)
'black hearts concealing terrible crimes' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'hearts' becomes 'h' (abbreviation used in card games e.g. bridge).
'concealing' indicates putting letters inside.
'terrible' indicates anagramming the letters.
'b'+'h'='bh'
'crimes' anagrammed gives 'esmirc'.
'bh' going around 'esmirc' is 'BESMIRCH'.
(Other definitions for besmirch that I've seen before include "dull" , "run down" , "Sully" , "Damage (a reputation)" , "Foul" .)