They go round or up over hills (6)

'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' shows that the letters should be reversed in order (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' backwards is 'ro'.
'ro'+'tors'='ROTORS'

'round' acts as a link.

Can you help me to learn more?