Falls once more upending rear, oddly (7)
I believe the answer is:
niagara
'falls' is the definition.
(Niagara Falls)
'once more upending rear oddly' is the wordplay.
'once more' becomes 'again' (synonyms).
'upending' says the letters should be written in reverse.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternate letters of 'rear' are 'ra'.
'again' back-to-front is 'niaga'.
'niaga'+'ra'='NIAGARA'
(Other definitions for niagara that I've seen before include "Great waterfalls on Canadian-US border" , "Great waterfalls on border of USA and Canada" , "They fall in North America" , "Great falls on border of Canada and USA" , "A lot of falling water" .)