A rigid support so revolutionary incidentally (7)
I believe the answer is:
apropos
'incidentally' is the definition.
(I've seen this before)
'a rigid support so revolutionary' is the wordplay.
'rigid support' becomes 'prop' (I've seen this before).
'revolutionary' says the letters should be written in reverse.
'so' in reverse letter order is 'os'.
'a'+'prop'+'os'='APROPOS'
(Other definitions for apropos that I've seen before include "As regards" , "With regard to" , "Poor sap (anag) - to the purpose" , "Poor sap, being to the point" , "Concerned with" .)