They go round or up over hills (6)
I believe the answer is:
rotors
'they go' is the definition.
Although both the answer and 'go' are nouns, they are not in the same form of the noun.
'or up over hills' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'hills' becomes 'tors' (tor is a kind of hill).
'or' in reverse letter order is 'ro'.
'ro'+'tors'='ROTORS'
'round' acts as a link.
Can you help me to learn more?
(Another definition for rotors that I've seen is " Turning parts of a device".)