All round, the London one has a focus of attention (3)
I believe the answer is:
eye
'attention' is the definition.
(I know that eye is a type of attention)
'all round the london one has a focus' is the wordplay.
'all round' says the letters should be written in reverse (all the words are reversed).
'the london one' becomes 'ye' (I can't explain this - if you can you should believe this answer much more).
'has' is a charade indicator (letters next to each other).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'focus' indicates the central letters (focus can mean the centre of something).
The middle letter of 'per' is 'e'.
'ye' written backwards gives 'ey'.
'ey'+'e'='EYE'
'of' is the link.
Can you help me to learn more?
(Other definitions for eye that I've seen before include "It accommodates a pupil" , "Centre of storm" , "Observer" , "'Public' or 'private' body part" , "Twenty four ac emphasises it" .)