Contemptuous and essentially conceited group of criminals (9)
I believe the answer is:
withering
'contemptuous' is the definition.
(thesaurus)
'and essentially conceited group of criminals' is the wordplay.
'and' becomes 'with'.
'essentially' says to take the centre.
'group of criminals' becomes 'ring'.
The central letter of 'conceited' is 'e'.
'with'+'e'+'ring'='WITHERING'
(Other definitions for withering that I've seen before include "Devastating" , "Supercilious" .)