Course of action for cops, mostly unknown (6)
I believe the answer is:
policy
'course of action' is the definition.
(I've seen this before)
'cops mostly unknown' is the wordplay.
'cops' becomes 'police' (cops can informally mean the police).
'mostly' means to remove the last letter (most but not all of the word).
'unknown' becomes 'y' (algebra).
'police' with its last letter removed is 'polic'.
'polic'+'y'='POLICY'
'for' acts as a link.
(Other definitions for policy that I've seen before include "Honesty is said to be the best one" , "(E.g. government) line" , "Set of principles - plan" , "Course of action proposed in a political manifesto" , "Plan of action adopted by an individual or group" .)