Very ace sink promoted in flat (5)
I believe the answer is:
vapid
'flat' is the definition.
('vapid' can be a synonym of 'flat')
'very ace sink promoted' is the wordplay.
'very' becomes 'v' (abbreviation).
'ace' becomes 'a'.
'sink' becomes 'dip' ('dip' can be a synonym of 'sink'**).
'promoted' says the letters should be written in reverse (going up, in a down clue).
'dip' back-to-front is 'pid'.
'v'+'a'+'pid'='VAPID'
'in' acts as a link.
(Other definitions for vapid that I've seen before include "Dull, insipid, flat" , "Dull and tasteless" , "Offering no stimulation" , "Lacking flavour" , "no interest" .)