Very keen to attack (3,4)
I believe the answer is:
set upon
'attack' is the definition.
(I know that set upon is a type of attack)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for set upon that I've seen before include "Attack" , "Mug" , "Attacked" , "Determined to do (something)" .)