Quiet force keeps returning fire for officer (7)
I believe the answer is:
Here is my best explanation:
'officer' is the definition.
'quiet force keeps returning fire' is the subsidiary indication.
'quiet' becomes 'sh' (**).
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'keeps' is an insertion indicator.
'returning' says the letters should be written backwards.
'fire' backwards is 'erif'.
'shf' enclosing 'erif' is 'sheriff'.
'for' is the link.
(Other definitions for sheriff that I've seen before include "Crown's county officer", "Officer of the law", "US law enforcement officer", "Law officer", "Local officer", "Chief Crown representative in a county", "Scottish judge", "Crown's chief executive in a county", "Head of the posse" and "US law-enforcement officer".)