One best placed at the summit (4)
I believe the answer is:
atop
'at the summit' is the definition.
(I've seen this before)
'one best' is the wordplay.
'one' becomes 'a'.
'best' becomes 'top' (similar in meaning).
'a'+'top'='ATOP'
'placed' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for atop that I've seen before include "Upon" , "On a high" , "Positioned on" , "Covering" , "On the peak" .)