Duke gets poor return for flag (5)
I believe the answer is:
droop
'flag' is the definition.
(I know that flag can be written as droop)
'duke gets poor return' is the wordplay.
'duke' becomes 'd' (abbreviation).
'gets' is a charade indicator (letters next to each other).
'return' is a reversal indicator.
'poor' in reverse letter order is 'roop'.
'd'+'roop'='DROOP'
'for' acts as a link.
(Other definitions for droop that I've seen before include "sink" , "lose heart" , "male problem?" , "Grow faint" , "Sag downwards" .)