Thunder a bit upsetting? Nonsense! (8)
I believe the answer is:
claptrap
'nonsense' is the definition.
(informal term meaning nonsense or rubbish)
'thunder a bit upsetting?' is the wordplay.
'thunder' becomes 'clap' (I am not sure about this - if you are sure you should believe this answer much more).
'a bit' becomes 'part' (both can mean a piece of something).
'upsetting?' is a reversal indicator (I've seen 'upsetting' mean this).
'part' back-to-front is 'trap'.
'clap'+'trap'='CLAPTRAP'
Can you help me to learn more?
(Other definitions for claptrap that I've seen before include "Pretentious talk used to win applause" , "Bull" , "poppycock" , "Cobblers" , "Applaud snare? - that's pretentious nonsense" .)