Bad day to raise flag (5)
I believe the answer is:
droop
'flag' is the definition.
(I know that flag can be written as droop)
'bad day to raise' is the wordplay.
'bad' becomes 'poor' (similar in meaning).
'day' becomes 'd' (abbreviation for day).
'to raise' is a reversal indicator (in a down clue, letters go up).
'poor'+'d'='poord'
'poord' back-to-front is 'DROOP'.
(Other definitions for droop that I've seen before include "Sag, hang down" , "Sag, wilt" , "Flag" , "languish" , "sink" .)