Don't fail to find a way through the mountains (4)
I believe the answer is:
'don't fail' is the definition.
'pass' can be an answer for 'fail' (thesaurus). I'm not certain of the 'don't' bit.
'a way through the mountains' is the wordplay.
'a way' becomes 's' (Compass direction - South).
'through' means one lot of letters goes inside another (inserted letters go through the word).
'the mountains' becomes 'pas' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
's' placed into '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" .)