Most challenging of peaks: night before get some sleep (7)
I believe the answer is:
everest
'most challenging of peaks' is the definition.
(highest mountain)
'night before get some sleep' is the wordplay.
'night before' becomes 'eve' (eve can mean the night before something).
'get' says to put letters next to each other.
'some sleep' becomes 'rest' (synonyms).
'eve'+'rest'='EVEREST'
(Other definitions for everest that I've seen before include "Mountain on the Nepal-Tibet border" , "Mountain named after a British surveyor general of India" , "Great achievement" , "Huge achievement - highest climb" , "High peak" .)