Duke gets poor return for flag (5)
I believe the answer is:
droop
'flag' is the definition.
(synonyms)
'duke gets poor return' is the wordplay.
'duke' becomes 'd' (abbreviation).
'gets' says to put letters next to each other.
'return' says the letters should be written backwards.
'poor' written backwards gives '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" .)