Organise a run to the mountains (7)
I believe the answer is:
arrange
'organise' is the definition.
(arranging is a kind of organizing)
'a run to the mountains' is the wordplay.
'a run' becomes 'ar' (I've seen this in other clues).
'to the mountains' becomes 'range' (I've seen this before).
'ar'+'range'='ARRANGE'
(Other definitions for arrange that I've seen before include "Put in order - organise" , "Place in order" , "Put in order; plan" , "Marshal" , "Dispose" .)