Falls during war against uprising (7)
I believe the answer is:
niagara
'falls' is the definition.
(Niagara Falls)
'during war against uprising' is the wordplay.
'during' indicates a hidden word.
'uprising' says the letters should be written in reverse.
'aragain' can be found hidden inside 'war against'.
'aragain' back-to-front is 'NIAGARA'.
(Other definitions for niagara that I've seen before include "1953 Marilyn Monroe film noir thriller" , "Horseshoe, American and Bridal Veil Falls, collectively" , "It falls in North America" , "it's quite a height" , "Famous Falls once walked by Blondin" .)