Staunch supporter of extremely shady organisation (6)
I believe the answer is:
system
'organisation' is the definition.
(synonyms)
'staunch supporter of extremely shady' is the wordplay.
'staunch' becomes 'stem' (synonyms).
'supporter of' is a charade indicator (letters next to each other) (supports the letters above in a down clue).
'extremely' suggests removing the centre (only the extremes of the word are used).
'shady' with its centre removed is 'sy'.
'stem' put after 'sy' is 'SYSTEM'.
(Other definitions for system that I've seen before include "Arrangement of things working together" , "A complete process" , "My sets (anag.)" , "Method, set of connected things" , "Organised means of work" .)