Definition: "Slander"

This is a charade - look for parts of the clue that can be broken down into separate components that join together to form the answer.

Break down 'heartlessly audacious' by removing the middle letters, then combine this with a phrase meaning 'in itself' to build your answer.

Answer: ASPERSE | A charade of AS (from 'heartlessly AudaciouS' - removing the middle letters) plus PER SE ('in itself') = ASPERSE