Artist working in the London police (5)
I believe the answer is:
monet
'artist' is the definition.
(painter Claude Monet)
'working in the london police' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'in the' means one lot of letters goes inside another.
'london police' becomes 'Met' (London Metropolitan Police).
'on' inserted into 'met' is 'MONET'.
(Other definitions for monet that I've seen before include "French impressionist (Waterlilies)" , "French Impressionist with famous garden" , "'Water Lilies' 13ac" , "Giverny painter" , "he painted Rouen Cathedral" .)