Polish cop arrests free running enthusiast finally (7)
I believe the answer is:
perfect
'polish' is the definition.
(I know that polish is a more specific form of the action perfect)
'cop arrests free running enthusiast finally' is the wordplay.
'cop' becomes 'pc' ().
'arrests' indicates putting letters inside (inserted letters are contained or arrested).
'running' indicates an anagram (the letters run around to different places).
'finally' suggests the final letters.
The last letter of 'enthusiast' is 't'.
'free' with letters rearranged gives 'erfe'.
'pc' placed around 'erfe' is 'perfec'.
'perfec'+'t'='PERFECT'
(Other definitions for perfect that I've seen before include "Ideal, without flaw" , "Without blemish" , "Ideal, flawless" , "Mature" , "Flawless - finish" .)