The Latin student's farewell to natural depression (4)
I believe the answer is:
vale
This might be a double definition.
'the latin student's farewell' is the first definition.
'vale' can be an answer for 'farewell' ('vale' is a Latin farewell). I am not certain of the 'the latin student's' bit.
'natural depression' is the second definition.
(vale is a kind of natural depression)
'to' acts as a link.
Can you help me to learn more?
(Other definitions for vale that I've seen before include "Valley" , "Long depression containing a river" , "Trough between ridges" , "Glen; goodbye!" , "Maida -, N. London area; farewell (archaic)" .)