The night before, slack off on the mountain (7)
I believe the answer is:
everest
'mountain' is the definition.
(Mount Everest)
'the night before slack off on the' is the wordplay.
'the night before' becomes 'eve' (eve can mean the night before something).
'slack off' becomes 'res' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' says to put letters next to each other.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'eve'+'res'+'t'='EVEREST'
Can you help me to learn more?
(Other definitions for everest that I've seen before include "Where Hillary was" , "A mountain to climb" , "Mountain named after a British surveyor general of India" , "Himalayan peak" , "Huge achievement - highest climb" .)