Four-fifths of extremely popular London area (4)
I believe the answer is:
soho
'london area' is the definition.
(I've seen this before)
'four-fifths of extremely popular' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is found within the answer.
The remaining letters 'sho' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for soho that I've seen before include "(Formerly) disreputable area of London" , "Colourful district in central London" , "Colourful London show-biz district" , "London area renowned for nightlife" , "London's nightlife centre" .)