At the summit, of course, there's a head (4)
I believe the answer is:
atop
'at the summit of' is the definition.
(I've seen this before)
'there's a head' is the wordplay.
'there's' means one lot of letters go next to another.
'head' becomes 'top' (synonyms).
'a' next to 'top' is 'ATOP'.
'course' acts as a 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 "On (poetic)" , "At the apex" , "Situated on" , "Upon" , "Surmounting" .)