A drop of rain once more elevated river (7)
I believe the answer is:
niagara
'river' is the definition.
(I know that Niagara is a river)
'a drop of rain once more elevated' is the wordplay.
'drop of' suggests taking the first letters.
'once more' becomes 'again' (synonyms).
'elevated' shows that the letters should be reversed in order (in a down clue, letters go up).
The first letter of 'rain' is 'r'.
'a'+'r'+'again'='aragain'
'aragain' written backwards gives 'NIAGARA'.
(Other definitions for niagara that I've seen before include "They fall in North America" , "Great falls on border of Canada and USA" , "N American river and Falls" , "Famous Falls once walked by Blondin" , "Great waterfalls on US-Canada border" .)