Policeman quietly infiltrating fashionable zone (9)
I believe the answer is:
inspector
'policeman' is the definition.
(inspector is a kind of policeman)
'quietly infiltrating fashionable zone' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'infiltrating' indicates putting letters inside.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'zone' becomes 'sector' (I've seen this before**).
'in'+'sector'='insector'
'p' going into 'insector' is 'INSPECTOR'.
(Other definitions for inspector that I've seen before include "one to check" , "reviewer" , "controller" , "Lestrade for instance" , "Police officer" .)