Upsetting shock makes one crazy (4)
I believe the answer is:
nuts
'one crazy' is the definition.
(thesaurus)
'upsetting shock' is the wordplay.
'upsetting' says the letters should be written in reverse.
'shock' becomes 'stun' ('stun' can be a synonym of 'shock').
'stun' written backwards gives 'NUTS'.
'makes' acts as a link.
(Other definitions for nuts that I've seen before include "Eg, filberts, hazels" , "Delivers 24 [HEADBUTT]" , "Bolt securers" , "Fruit - bananas!" , "deprived of one's wits" .)