Police officer quietly delayed writing (11)
I believe the answer is:
copperplate
'writing' is the definition.
(copperplate is a kind of writing)
'police officer quietly delayed' is the wordplay.
'police officer' becomes 'copper' (copper is a kind of police officer**).
'quietly' becomes 'p' (p for piano and quiet).
'delayed' becomes 'late' (I've seen this before).
'copper'+'p'+'late'='COPPERPLATE'
(Other definitions for copperplate that I've seen before include "good hand" , "Ornate writing" , "hand: that's fine" , "sort of hand" .)