Bill joins me at the summit (4)
I believe the answer is:
acme
'the summit' is the definition.
('acme' can be a synonym of 'summit')
'bill joins me' is the wordplay.
'bill' becomes 'ac' (short for account).
'joins' means one lot of letters go next to another.
'ac'+'me'='ACME'
'at' is the link.
(Other definitions for acme that I've seen before include "Summit, crowning point" , "Summit - zenith" , "Height of achievement" , "'Highest point, apex (4)'" , "Highest level or degree attainable" .)