An arresting thing to say (4,2)
I believe the answer is:
stop it
'an arresting thing to say' is the definition.
I know nothing about this answer so I can't judge whether this works.
'arresting thing to say' is the wordplay.
'arresting' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'thing' becomes 'it'.
'to' means one lot of letters go next to another (I've seen this in other clues).
'say' becomes 'top'.
'it' put after 'top' is 'topit'.
's'+'topit'='STOP IT'
Can you help me to learn more?
(Other definitions for stop it that I've seen before include "Don't" , "Desist!" .)