Fraud can force revolution, every now and then. Agree? (6)
I believe the answer is:
concur
'agree?' is the definition.
('concur' can be a synonym of 'agree')
'fraud can force revolution' is the wordplay.
'fraud' becomes 'con' (con is a kind of fraud).
'can force' becomes 'ruc' (I can't explain this - if you can you should give a lot more credence to this answer).
'revolution' says the letters should be written backwards.
'ruc' back-to-front is 'cur'.
'con'+'cur'='CONCUR'
'every now and then' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for concur that I've seen before include "meet" , "See eye to eye" , "share the same opinion" , "Agree, assent" , "Be in accord" .)