Top police force take a nosedive (7)
I believe the answer is:
plummet
'a nosedive' is the definition.
(I've seen this in another clue)
'top police force' is the wordplay.
'top' becomes 'plum' (plum can mean excellent or top-class).
'police force' becomes 'met' (short for Metropolitan Police Service).
'plum'+'met'='PLUMMET'
'take' is the link.
(Other definitions for plummet that I've seen before include "fall sharply" , "Plunge headlong" , "Drop at speed" , "Fall headlong" , "Drop sharply" .)