Posh fellows in crime, packing (8)
I believe the answer is:
stuffing
'packing' is the definition.
(thesaurus)
'posh fellows in crime' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'fellows' becomes 'ff'.
'in' indicates putting letters inside.
'crime' becomes 'sting' (sting is a kind of crime**).
'u'+'ff'='uff'
'uff' placed inside 'sting' is 'STUFFING'.
(Other definitions for stuffing that I've seen before include "[TRIMMING]" , "See 1" , "good beating" , "Turkey filling" , "Forcemeat" .)