Vague hunch about policeman (10)
I believe the answer is:
indistinct
'vague' is the definition.
(thesaurus)
'hunch about policeman' is the wordplay.
'hunch' becomes 'instinct' (I've seen this before).
'about' indicates putting letters inside.
'policeman' becomes 'di' (I've seen this in other clues).
'instinct' enclosing 'di' is 'INDISTINCT'.
(Other definitions for indistinct that I've seen before include "Not clearly defined" , "Obscure" , "Blurred" .)