Police officer going over a lightweight chart (7)
I believe the answer is:
diagram
'chart' is the definition.
(I've seen this before)
'police officer going over a lightweight' is the wordplay.
'police officer' becomes 'di' (abbreviation for Detective Inspector).
'going over' says to put letters next to each other.
'lightweight' becomes 'gram'.
'di'+'a'+'gram'='DIAGRAM'
(Other definitions for diagram that I've seen before include "Schematic drawing" , "scheme" , "Line drawing" , "A grid, Ma? (Anag)" , "picture" .)