Happen to be facing revolutionary change (6)
I believe the answer is:
betide
'happen to' is the definition.
('betide' can be a synonym of 'happen to')
'be facing revolutionary change' is the wordplay.
'facing' means one lot of letters go next to another.
'revolutionary' says the letters should be written backwards.
'change' becomes 'edit' (editing is a kind of changing**).
'edit' written backwards gives 'tide'.
'be'+'tide'='BETIDE'
(Other definitions for betide that I've seen before include "Happen to (arch)" , "Befall" , "Woe - -" .)