Drop police force by choice (7)
I believe the answer is:
plummet
'drop' is the definition.
(plummeting is a kind of dropping)
'police force by choice' is the wordplay.
'police force' becomes 'met' (short for Metropolitan Police Service).
'by' means one lot of letters go next to another.
'choice' becomes 'plum' (I've seen this before).
'met' put after 'plum' is 'PLUMMET'.
(Other definitions for plummet that I've seen before include "fall sharply" , "Drop straight down" , "Descend" , "Drop at speed" , "Fall rapidly" .)