Lawful arrest by pollice officer after a carnival (1,4,3)
I believe the answer is:
a fair cop
'lawful arrest by pollice' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'officer after a carnival' is the wordplay.
I cannot really see how this works, but
'officer' could be 'cop' (cop is a kind of officer) and 'cop' is located in the answer.
'a' is present in the answer.
'carnival' could be 'fair' (synonyms) and 'fair' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for a fair cop that I've seen is " Deserved arrest or discovery as by an equitable policeman".)