The sort of fathead you want out of London (4)
I believe the answer is:
loon
'london' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'the sort of fathead you want out of' is the wordplay.
'the sort of fathead' becomes 'loo' (I can't justify this - if you can you should believe this answer much more).
'you' becomes 'ye' (historical version of 'you').
'want' becomes 'yen' (I've seen this in another clue).
'out of' suggests deleting specific letters.
'yen' with 'ye' removed is 'n'.
'loo'+'n'='LOON'
Can you help me to learn more?
(Other definitions for loon that I've seen before include "Fool" , "Noddy" , "Diver; silly person" , "Fish-eating diving bird" , "N American diving bird" .)