Summit conference's opening before holiday (5)
I believe the answer is:
crest
'summit' is the definition.
('crest' can be a synonym of 'summit')
'conference's opening before holiday' is the wordplay.
'opening' suggests taking the first letters.
'before' is a charade indicator (letters next to each other).
'holiday' becomes 'rest' (I've seen this before).
The initial letter of 'conferences' is 'c'.
'c'+'rest'='CREST'
(Other definitions for crest that I've seen before include "Top of a ridge or wave" , "Top of a ridge/wave" , "Plume on a helmet" , "Highest point" , "Surmount" .)