Defeat decisively, right away (4)
I believe the answer is:
rout
'defeat' is the definition.
(rout is a kind of defeat)
'decisively right away' is the wordplay.
'decisively right' becomes 'r' (common abbreviation. I am not sure about the 'decisively' bit.**).
'away' becomes 'out' ('out' can be a synonym of 'away').
'r'+'out'='ROUT'
Can you help me to learn more?
(Other definitions for rout that I've seen before include "Vanquish; thrash" , "Easy victory (or defeat)" , "Total(ly) defeat" , "Defeat comprehensively" , "Overwhelming victory" .)