Collusion of sanctimonious blue supporting Tories (10)
I believe the answer is:
conspiracy
'collusion' is the definition.
(thesaurus)
'sanctimonious blue supporting tories' is the wordplay.
'sanctimonious' becomes 'pi' (British abbreviation for pious).
'blue' becomes 'racy' (synonyms).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'tories' becomes 'cons' (short for Conservatives).
'pi'+'racy'='piracy'
'piracy' put after 'cons' is 'CONSPIRACY'.
'of' acts as a link.
(Other definitions for conspiracy that I've seen before include "This is intriguing" , "Joint action undertaken secretly" , "Secret plan" , "intrigue" , "that involves plotting" .)