Many think about summit on Everest, primarily (8)
I believe the answer is:
multiple
'many' is the definition.
(similar in meaning)
'think about summit on everest primarily' is the wordplay.
'think' becomes 'mull' (to mull something over is to think it over).
'about' is an insertion indicator.
'summit' becomes 'tip' (synonyms).
'on' is a charade indicator (letters next to each other).
'primarily' suggests taking the first letters.
The initial letter of 'everest' is 'e'.
'mull' placed around 'tip' is 'multipl'.
'multipl'+'e'='MULTIPLE'
(Other definitions for multiple that I've seen before include "Having many parts or components" , "Lit plume (anag)" , "Manifold; numerous" , "Numerous and often varied" , "Having several parts" .)