Falls once more going over the border of Rwanda (7)
I believe the answer is:
niagara
'falls' is the definition.
(Niagara Falls)
'once more going over the border of rwanda' is the wordplay.
'once more' becomes 'again' (synonyms).
'going over' says the letters should be written in reverse.
'the border of' suggests removing the centre (I've seen 'borders of' mean this).
'rwanda' with its middle removed is 'ra'.
'again' in reverse letter order is 'niaga'.
'niaga'+'ra'='NIAGARA'
(Other definitions for niagara that I've seen before include "1953 Marilyn Monroe film noir thriller" , "River linking Lakes Ontario and Erie" , "Famous waterfalls" , "Waterfall at Iran aga" , "Famous Falls once walked by Blondin" .)