One coming out from mountain box departs westwards
I believe the answer is:
debutante
'one coming out from mountain box departs westwards' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one coming out from mountain box' is the wordplay.
'one coming out' becomes 'debut' (I've seen this before).
'from' is a charade indicator (letters next to each other).
'mountain box' becomes 'ante' (I can't explain this - if you can you should believe this answer much more).
'debut'+'ante'='DEBUTANTE'
Can you help me to learn more?
(Other definitions for debutante that I've seen before include "Society girl" , "Young lady making her first appearance in society" , "one about to come out" , "Maiden speaker" , "young lady coming out" .)