... to arrest a small child (6)
I believe the answer is:
nipper
'small child' is the definition.
(I've seen this before)
'to arrest a' is the wordplay.
'to arrest' becomes 'nip' (I can't justify this - if you can you should give a lot more credence to this answer).
'a' becomes 'per'.
'nip'+'per'='NIPPER'
Can you help me to learn more?
(Other definitions for nipper that I've seen before include "A pincer or a young boy" , "Young person (with claws?)" , "Kid - claw" , "Small boy - or crab?" , "Child (slang); one that 19s" .)