County constabulary tailed streaker (5)
I believe the answer is:
comet
'tailed streaker' is the definition.
(celestial object which streaks across the sky)
'county constabulary' is the wordplay.
'county' becomes 'co'.
'constabulary' becomes 'Met' (short for the Metropolitan Police Service).
'co'+'met'='COMET'
(Other definitions for comet that I've seen before include "Halley's phenomenon in space" , "Tailed space traveller" , "bright star" , "It orbits the sun, with bright centre and long tail" , "Small extraterrestrial body" .)