Inform on these lethal extremists (4)
I believe the answer is:
tell
'inform' is the definition.
(I know that tell is a more specific form of the action inform)
'on these lethal extremists' is the wordplay.
'on' says to put letters next to each other.
'these' becomes 'te' (I can't explain this - if you can you should believe this answer much more).
'extremists' means to remove the middle letters (letters on the extreme ends of the word).
'lethal' with its middle taken out is 'll'.
'te' next to 'll' is 'TELL'.
Can you help me to learn more?
(Other definitions for tell that I've seen before include "William ..., skilled Swiss archer" , "Rossini composed opera 'William ....'" , "William . . . . was a legendary Swiss archer" , "Instruct" , "his hitting the apple?" .)