A bit upset by thunder? Nonsense! (8)
I believe the answer is:
claptrap
'nonsense' is the definition.
(informal term meaning nonsense or rubbish)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for claptrap that I've seen before include "Contrived but foolish talk" , "Meaningless nonsense" , "Meaningless, insincere or pompous talk" , "Pretentious talk used to win applause" , "Applaud snare? - that's pretentious nonsense" .)