Greek character with pistol upset police (8)
I believe the answer is:
regulate
'police' is the definition.
(I've seen this before)
'greek character with pistol upset' is the wordplay.
'greek character' becomes 'eta' (letter from the Greek alphabet).
'with' means one lot of letters go next to another.
'pistol' becomes 'luger' (I have seen 'German pistol' mean 'luger' so perhaps 'pistol' could also mean 'luger').
'upset' shows that the letters should be reversed in order.
'eta'+'luger'='etaluger'
'etaluger' in reverse letter order is 'REGULATE'.
(Other definitions for regulate that I've seen before include "Ate gruel (anag.)" , "Control or direct by means of rules and procedures" , "Bring order" , "Control, supervise" , "Ensure conformity with the rules" .)