Dry lecture follows hoisting of flag (9)
I believe the answer is:
evaporate
'dry' is the definition.
(thesaurus)
'lecture follows hoisting of flag' is the wordplay.
'lecture' becomes 'orate' (I've seen this before).
'follows' is a charade indicator (letters next to each other).
'hoisting of' shows that the letters should be reversed in order (in a down clue, letters hoisted up).
'flag' becomes 'pave' (as in to pave with flagstones).
'pave' in reverse letter order is 'evap'.
'orate' after 'evap' is 'EVAPORATE'.
(Other definitions for evaporate that I've seen before include "Turn into vapour, lose moisture" , "Turn from liquid to vapour" , "go a stage further?" , "Desiccate, dry up" , "Go from liquid to vapour" .)