Send on description of bellicose duke (7)
I believe the answer is:
forward
'send on' is the definition.
(as in forwarding a letter on)
'description of bellicose duke' is the wordplay.
'description of bellicose' becomes 'for war' (in favour of war).
'duke' becomes 'd' (abbreviation).
'forwar'+'d'='FORWARD'
(Other definitions for forward that I've seen before include "Promote" , "Send on; football player" , "Presumptuous" , "Straight ahead" , "Officious" .)