Harangue under inverted flag and disappear (9)
I believe the answer is:
evaporate
'disappear' is the definition.
(I've seen this in another clue)
'harangue under inverted flag' is the wordplay.
'harangue' becomes 'orate' (I've seen this before).
'under' says to put letters next to each other (in a down clue, some letters go below others).
'inverted' says the letters should be written backwards.
'flag' becomes 'pave' (as in to pave with flagstones).
'pave' backwards is 'evap'.
'orate' after 'evap' is 'EVAPORATE'.
'and' acts as a link.
(Other definitions for evaporate that I've seen before include "go a stage further?" , "Turn from liquid to vapour" , "Melt away" , "Disappear into thin air" , "Desiccate, dry up" .)