Body of policemen lean on pornographic business (4,4,4)
I believe the answer is:
thin blue line
'body of policemen' is the definition.
'lean on pornographic business' is the wordplay.
'lean' becomes 'thin' (synonyms).
'on' says to put letters next to each other.
'pornographic' becomes 'blue' (I've seen this in another clue).
'business' becomes 'line' (as in someone's line of work).
'thin'+'blue'+'line'='THIN BLUE LINE'
(Other definitions for thin blue line that I've seen before include "Police holding back a riot?" , "police maintaining order?" .)