Event in which police backed soldiers spontaneously (9)
I believe the answer is:
extempore
'spontaneously' is the definition.
(I know that spontaneously can be written as extempore)
'event in which police backed soldiers' is the wordplay.
'event' becomes 'expo' (exposition).
'in which' is an insertion indicator.
'police' becomes 'met' (London Metropolitan Police).
'backed' is a reversal indicator.
'soldiers' becomes 're' (Royal Engineers).
'met' back-to-front is 'tem'.
'expo' going around 'tem' is 'extempo'.
'extempo'+'re'='EXTEMPORE'
(Other definitions for extempore that I've seen before include "unprepared" , "without warning" , "Not prepared" , "Spoken off the cuff" , "Dashed off" .)