Form of words used in making an arrest (4,2)
I believe the answer is:
stop it
'an arrest' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'form of words used in making' is the wordplay.
'form of words' becomes 'stop' (I am not sure about this - if you are sure you should believe this answer much more).
'used in' says to take the centre (the letters used on the inside of the word).
'making' becomes 'inciting' (inciting is a kind of making).
The middle of 'inciting' is 'it'.
'stop'+'it'='STOP IT'
Can you help me to learn more?
(Other definitions for stop it that I've seen before include "Don't" , "Desist!" .)