What Mussolini's supporters were to bring about (7)
I believe the answer is:
produce
'to bring about' is the definition.
('produce' can be a synonym of 'bring about')
'what mussolini's' is the wordplay.
'what' becomes 'pro' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'mussolini' becomes 'duce' (I've seen this before).
'pro'+'duce'='PRODUCE'
'supporters were' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
(Other definitions for produce that I've seen before include "Bring forth or yield" , "Create, manufacture" , "Make, manufacture" , "Fresh fruit and veg for market - crop due (anag)" , "Make, create" .)