Highest mountain in the Alps (4,5)
I believe the answer is:
mont blanc
'highest mountain in the alps' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for mont blanc that I've seen before include "Highest mountain in the Alps" , "peak form" , "Mountain in 6" , "Fancy pen" , "High spot in France (and partly 18)" .)