Castigate about terrible evil (6)
I believe the answer is:
revile
'castigate' is the definition.
The answer and definition can be both to do with communicating as well as being verbs in their base form.
Maybe they are linked in a way I don't understand?
'about terrible evil' is the wordplay.
'about' becomes 're' (common abbreviation for 'with reference to').
'terrible' indicates anagramming the letters.
'evil' with letters rearranged gives 'vile'.
're'+'vile'='REVILE'
Can you help me to learn more?
(Other definitions for revile that I've seen before include "Run down" , "Upbraid or abuse" , "Abusively criticise" , "Criticise - abuse" , "Upbraid, berate" .)