Crazy upper-class twit discharged in US city (5)
I believe the answer is:
nutty
'crazy' is the definition.
(I've seen this before)
'upper-class twit discharged in us city' is the wordplay.
'upper-class' becomes 'u' (similar in meaning).
'discharged' says to hollow out the word (remove centre letters) (the letters inside the word released or discharged).
'in' is an insertion indicator.
'us city' becomes 'NY' (abbreviation for New York).
'twit' with its centre taken out is 'tt'.
'u'+'tt'='utt'
'utt' inserted inside 'ny' is 'NUTTY'.
(Other definitions for nutty that I've seen before include "Mad - with small pieces of coal" , "Eccentric - like nougat?" , "passionate" , "Crackers" , "Like, eg, brazils" .)