Remarkable events men phone up about at summit (9)
I believe the answer is:
phenomena
'remarkable events' is the definition.
(I've seen this before)
'men phone up about at summit' is the wordplay.
'up' indicates an anagram (letters get chucked up).
'at summit' indicates taking the first letters (the top letter or 'summit').
The first letter of 'about' is 'a'.
'men'+'phone'='menphone'
'menphone' anagrammed gives 'phenomen'.
'phenomen'+'a'='PHENOMENA'
(Other definitions for phenomena that I've seen before include "wondrous things" , "Events that may be observed" , "Things sensed" , "Unusual developments" , "Curious events" .)