It goes round or up over a hill (5)
I believe the answer is:
rotor
'it goes round' is the definition.
(I've seen this before)
'or up over a hill' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'a hill' becomes 'tor' (tor is a kind of hill).
'or' backwards is 'ro'.
'ro'+'tor'='ROTOR'
(Other definitions for rotor that I've seen before include "Part of electric motor" , "Revolving part of a machine, a palindrome" , "It lifts helicopter" , "Hub with radiating blades" , "Revolving part of turbine" .)