Unruly as punks, one finally arrested comes quietly (6,2)

'comes quietly' is the definition.
(to sneak up on someone is to approach them quietly)

'unruly as punks one finally arrested' is the wordplay.
'unruly' is an anagram indicator.
'finally' indicates one should take the final letters.
'arrested' means one lot of letters goes inside another (arrest can mean to hold or imprison).
The final letter of 'one' is 'e'.
'as'+'punks'='aspunks'
'aspunks' anagrammed gives 'snaksup'.
'snaksup' going around 'e' is 'SNEAKS UP'.