Capital raised by duke for herald's tunic (6)
I believe the answer is:
tabard
'herald's tunic' is the definition.
(I've seen this before)
'capital raised by duke' is the wordplay.
'capital' becomes 'Rabat' (capital of Morocco).
'raised by' shows that the letters should be reversed in order.
'duke' becomes 'd' (abbreviation).
'rabat' in reverse letter order is 'tabar'.
'tabar'+'d'='TABARD'
'for' acts as a link.
(Other definitions for tabard that I've seen before include "Gilet" , "Knight's outer tunic" , "Jerkin; heraldic coat" , "Sleeveless jerkin" , "Historical tunic" .)