Police operation has run to a series (6)
I believe the answer is:
string
'to a series' is the definition.
(sequence of similar items or events)
'police operation has run' is the wordplay.
'police operation' becomes 'sting' (sting can mean a police operation).
'has' is an insertion indicator.
'run' becomes 'r' (cricket abbreviation).
'sting' going around 'r' is 'STRING'.
(Other definitions for string that I've seen before include "Group of racers" , "Thin piece of cord" , "Group of horses" , "Cord for a group of racehorses" , "might one be plucked?" .)