Reason some police use bad language (7)
I believe the answer is:
discuss
'reason' is the definition.
(discussing is a kind of reasoning)
'some police use bad language' is the wordplay.
'some police' becomes 'dis' (detective inspectors or DIs).
'use bad language' becomes 'cuss' (I've seen this before).
'dis'+'cuss'='DISCUSS'
(Other definitions for discuss that I've seen before include "Settle" , "Confer" , "Go into" , "Argue" , "Talk about (something)" .)