Powder found by detective arresting posh model (4)
I believe the answer is:
dust
'powder' is the definition.
(I've seen this in another clue)
'detective arresting posh model' is the wordplay.
'detective' becomes 'ds' (detective sergeant).
'arresting' is an insertion indicator.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'model' becomes 't' (Ford Model T car).
'ds' going around 'u' is 'dus'.
'dus'+'t'='DUST'
'found by' acts as a link.
(Other definitions for dust that I've seen before include "Fine dirt particles" , "Tiny particles of dirt, skin flakes etc" , "Do some housework - we all come to it!" , "Clean; fine powder" , "Clean - dirt" .)