Is furious. Could last for a long time (5)
I believe the answer is:
rages
'is furious' is the definition.
I can't tell whether this defines the answer.
'last for a long time' is the wordplay.
'last' indicates one should take the final letters.
'a long time' becomes 'ages' (ages can mean a very long time).
The last letter of 'for' is 'r'.
'r'+'ages'='RAGES'
'could' is the link.
Can you help me to learn more?
(Other definitions for rages that I've seen before include "gives anger vent" , "Passions" , "speaks as Hitler!" , "becomes furious" , "States of extreme anger" .)