Police once turned up in taxis at sunrise (5)
I believe the answer is:
stasi
'police' is the definition.
(I have seen 'secret police' mean 'stasi' so perhaps 'police' could also mean 'stasi')
'turned up in taxis at sunrise' is the wordplay.
'turned up' says the letters should be written in reverse.
'in' says the answer is hidden in the clue.
'isats' can be found hidden inside 'taxis at sunrise'.
'isats' backwards is 'STASI'.
'once' acts as a link.
(Other definitions for stasi that I've seen before include "East German Secret Police" , "Former German security service" , "Secret police of the former German Democratic Republic" , "old secret service" , "Former security force" .)