A Roman leader once more rises and falls (7)
I believe the answer is:
niagara
'falls' is the definition.
(Niagara Falls)
'a roman leader once more rises' is the wordplay.
'leader' says to take the initial letters.
'once more' becomes 'again' (synonyms).
'rises' is a reversal indicator.
The initial letter of 'roman' is 'r'.
'a'+'r'+'again'='aragain'
'aragain' in reverse letter order is 'NIAGARA'.
'and' acts as a link.
(Other definitions for niagara that I've seen before include "Famous 12" , "They fall in North America" , "Horseshoe, American and Bridal Veil Falls, collectively" , "Waterfall at Iran aga" , "Great falls on border of Canada and USA" .)