Critical intervention, the police initially stopping criminal activity (10)

'critical' is the definition.
(similar in meaning)

'intervention the police initially stopping criminal activity' is the wordplay.
'initially' suggests taking the first letters.
'stopping' means one lot of letters goes inside another (stopping can mean blocking a gap).
'criminal activity' becomes 'nicking' (I am not sure about this - if you are sure you should believe this answer much more).
The initial letters of 'intervention the police' is 'itp'.
'itp' placed inside 'nicking' is 'NITPICKING'.

Can you help me to learn more?