Weaken policeman with instrument (6)
I believe the answer is:
dilute
'weaken' is the definition.
(diluting is a kind of weakening)
'policeman with instrument' is the wordplay.
'policeman' becomes 'di' (detective inspector).
'with' means one lot of letters go next to another.
'instrument' becomes 'lute' (lute is a kind of instrument).
'di'+'lute'='DILUTE'
(Other definitions for dilute that I've seen before include "lost concentration" , "Reduce concentration of a liquid" , "Weaken by adding water" , "not neat" , "Tone down" .)