Shakespearean killer with tongue for slander (7)
I believe the answer is:
asperse
'for slander' is the definition.
(synonyms)
'shakespearean killer with tongue' is the wordplay.
'shakespearean killer' becomes 'asp' (I've seen this in another clue).
'with' says to put letters next to each other.
'tongue' becomes 'erse' (Erse is an example).
'asp'+'erse'='ASPERSE'
(Other definitions for asperse that I've seen before include "Slander or calumniate" , "See spar could slander oddly" , "Slander, spread false rumours about - as peers may?" , "Call names" , "Slander like Parsees" .)