Is furious right away - for a long time (4)
I believe the answer is:
ages
'long time' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ages that I've seen before include "Times" , "Donkey's years" , "(What seems) a very long time" , "Historical eras" , "Gets older" .)