A river once more rises and falls (7)
I believe the answer is:
niagara
'falls' is the definition.
(Niagara Falls)
'a river once more rises' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'once more' becomes 'again' (synonyms).
'rises' shows that the letters should be reversed in order.
'a'+'r'+'again'='aragain'
'aragain' written backwards gives 'NIAGARA'.
'and' acts as a link.
(Other definitions for niagara that I've seen before include "it's quite a height" , "US/Canadian falls" , "Famous 12" , "North American river with famous falls" , "Famous waterfalls" .)