Flag raised after millions desert (5)
I believe the answer is:
merit
'desert' is the definition.
(I've seen this before)
'flag raised after millions' is the wordplay.
'flag' becomes 'tire' (I've seen this in another clue).
'raised' shows that the letters should be reversed in order (going up in a down clue).
'after' says to put letters next to each other.
'millions' becomes 'm' (abbreviation).
'tire' back-to-front is 'erit'.
'erit' put after 'm' is 'MERIT'.
(Other definitions for merit that I've seen before include "distinction" , "Deserve, be worthy of" , "Excellence, worth" , "Excellence that deserves to be honoured" , "credit" .)