Mountain climbing with new intention (4)
I believe the answer is:
plan
'intention' is the definition.
(I've seen this in another clue)
'mountain climbing with new' is the wordplay.
'mountain' becomes 'alp' (the Alps are a mountain range).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'with' says to put letters next to each other.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'alp' written backwards gives 'pla'.
'pla'+'n'='PLAN'
(Other definitions for plan that I've seen before include "Scale drawing of a structure" , "Intend" , "Intention or decision about what one is going to do" , "Road map" , "Detailed proposal" .)