Crazy part of London (7)
I believe the answer is:
barking
'london' is the definition.
The definition and answer are not the same part of speech.
'crazy part' is the wordplay.
I cannot quite see how this works, but
'part' could be 'in' (synonyms) and 'in' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'of' is the link.
Can you help me to learn more?
(Other definitions for barking that I've seen before include "London suburb ..." , "Mad - town forming a London borough with Dagenham" , "Area of London - mad?" , "Dog may be" , "Completely mad" .)