Some chaps no longer at the top (4)

I believe the answer is:

apex

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'the top' is the definition.
(apex is a top or peak)

'some chaps no longer' is the wordplay.
'some chaps' becomes 'ap' (I can't explain this - if you can you should believe this answer much more).
'no longer' becomes 'ex' (prefix meaning former or onetime).
'ap'+'ex'='APEX'

'at' is the link.

Can you help me to learn more?

(Other definitions for apex that I've seen before include "Pointed top" , "Peak, vertex" , "Top point" , "Top, highest part" , "The highest point of something" .)

I've seen this clue in the Evening Standard.

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