First lady to settle on a mountain (7)
I believe the answer is:
everest
'a mountain' is the definition.
(I've seen this before)
'first lady to settle on' is the wordplay.
'first lady' becomes 'eve' (I've seen this before).
'to settle' becomes 'rest' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'eve'+'rest'='EVEREST'
(Other definitions for everest that I've seen before include "Mountain on the Nepal-Tibet border" , "High mountain" , "Himalyan peak" , "Mountain named after a British surveyor general of India" , "Himalayan peak" .)