Day before break for summit (7)
I believe the answer is:
everest
'summit' is the definition.
(Everest is an example)
'day before break' is the wordplay.
'day before' becomes 'eve' (I've seen this before).
'break' becomes 'rest' (resting is a kind of breaking).
'eve'+'rest'='EVEREST'
'for' is the link.
(Other definitions for everest that I've seen before include "A mountain to climb" , "Highest peak" , "Mountain on the Nepal-Tibet border" , "Where Hillary was" , "Highest Himalayan mountain" .)