Organise a run over series of mountains (7)
I believe the answer is:
arrange
'organise' is the definition.
(arranging is a kind of organizing)
'a run over series of mountains' is the wordplay.
'a run' becomes 'ar' (I've seen this in other clues).
'over' says to put letters next to each other.
'series of mountains' becomes 'range' (I've seen this before).
'ar'+'range'='ARRANGE'
(Other definitions for arrange that I've seen before include "Bring about or put in order" , "Put in order; plan" , "Put in neat order" , "Put into a proper order" , "Fix" .)