Duke gets inadequate return for flag (5)
I believe the answer is:
droop
'for flag' is the definition.
(synonyms)
'duke gets inadequate return' is the wordplay.
'duke' becomes 'd' (abbreviation).
'gets' means one lot of letters go next to another.
'inadequate' becomes 'poor' (synonyms).
'return' is a reversal indicator.
'poor' written backwards gives 'roop'.
'd'+'roop'='DROOP'
(Other definitions for droop that I've seen before include "be despondent" , "Grow faint" , "Sag downwards" , "Hang limply" , "Decline" .)