Law enforcers slim down policy (4,4,4)
I believe the answer is:
thin blue line
'law enforcers' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'slim down policy' is the wordplay.
'slim' becomes 'thin' (synonyms).
'down' becomes 'blue' (both can mean sad).
'policy' becomes 'line' (the party line is a party's policy on something).
'thin'+'blue'+'line'='THIN BLUE LINE'
Can you help me to learn more?
(Other definitions for thin blue line that I've seen before include "police maintaining order?" , "Police holding back a riot?" .)