About to relax upon summit (5)
I believe the answer is:
crest
'summit' is the definition.
(synonyms)
'about to relax' is the wordplay.
'about' becomes 'c' (circa abbreviation).
'to relax' becomes 'rest' (I've seen this before).
'c'+'rest'='CREST'
'upon' acts as a link.
(Other definitions for crest that I've seen before include "Top point of hill or wave" , "Summit ridge" , "Surmount" , "Crown, peak, summit" , "Top of a ridge/wave" .)