Don't fail to find a way through the mountains (4)
I believe the answer is:
pass
'don't fail' is the definition.
'pass' can be an answer for 'fail' (thesaurus). I'm unsure of the 'don't' bit.
'a way through the mountains' is the wordplay.
'a way' becomes 's' (Compass direction - South).
'through' indicates putting letters inside (inserted letters go through the word).
'the mountains' becomes 'pas' (I can't justify this - if you can you should believe this answer much more).
's' inserted within 'pas' is 'PASS'.
'to find' is the link.
Can you help me to learn more?
(Other definitions for pass that I've seen before include "Go by; mountain route" , "Get beyond" , "What a hooker may do" , "Narrow route through high mountain, like the Khyber" , "Elapse, go by" .)