Flattened garden ever rising, to some extent (6)
I believe the answer is:
evened
'flattened' is the definition.
(evened out or made uniform)
'garden ever rising to some extent' is the wordplay.
'to some extent' indicates a hidden word.
'rising' shows that the letters should be reversed in order (going up in a down clue).
'deneve' is hidden in the letters of 'garden ever'.
'deneve' reversed gives 'EVENED'.
(Other definitions for evened that I've seen before include "Levelled" , "Made smooth or equalised" , "Made smooth and regular" , "Made level" .)