Mountain lady to take a break (7)
I believe the answer is:
everest
'mountain' is the definition.
(Mount Everest)
'lady to take a break' is the wordplay.
'lady' becomes 'eve' (woman's name).
'to take a break' becomes 'rest' (to have a rest is to take a break).
'eve'+'rest'='EVEREST'
(Other definitions for everest that I've seen before include "Himalyan peak" , "One conquered in 1953" , "A mountain to climb" , "Mountain on the Nepal-Tibet border" , "Highest peak" .)