Many a blackguard joining fascist organisation (5)
I believe the answer is:
scads
'many' is the definition.
(scads of something means large amounts of it)
'a blackguard joining fascist organisation' is the wordplay.
'a blackguard' becomes 'cad' (both can mean a dishonourable person).
'joining' means one lot of letters goes inside another (inserted letters join the word).
'fascist organisation' becomes 'ss' (Nazi paramilitary organisation).
'cad' put into 'ss' is 'SCADS'.
(Other definitions for scads that I've seen before include "Mackerel-like fish" , "A lot" .)