A French copper with very loud siren, initially releases criminal (7)

'criminal' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'a french copper with very loud siren initially' is the wordplay.
'a french' becomes 'un' ('a' in French).
'copper' becomes 'cu' (Cu is the chemical symbol for copper).
'with' says to put letters next to each other.
'very loud' becomes 'ff' (musical abbreviation for 'fortissimo', very loud).
'initially' indicates taking the first letters.
The first letter of 'siren' is 's'.
'un'+'cu'+'ff'+'s'='UNCUFFS'

'releases' is the link.

Can you help me to learn more?