Is about to revolt, but flags (6)
I believe the answer is:
irises
'flags' is the definition.
(I know this)
'is about to revolt' is the wordplay.
'about' means one lot of letters goes inside another.
'to revolt' becomes 'rise' (I've seen this in another clue).
'is' enclosing 'rise' is 'IRISES'.
'but' acts as a link.
(Other definitions for irises that I've seen before include "Flags" , "Expensive painting" , "Coloured membranes in eyes, or showy flowers" , "Subject for van Gogh" , "blooms" .)