Falls back once more holding case of retsina... (7)
I believe the answer is:
niagara
'falls' is the definition.
(thesaurus)
'back once more holding case of retsina' is the wordplay.
'back' says the letters should be written in reverse.
'once more' becomes 'again' ('again' can be a synonym of 'once more').
'holding' means one lot of letters go next to another.
'case of' means to remove the middle letters (outsides of).
'retsina' with its centre taken out is 'ra'.
'again' written backwards gives 'niaga'.
'niaga'+'ra'='NIAGARA'
(Other definitions for niagara that I've seen before include "River forming part of US-Canada border" , "North American waterfalls" , "A lot of falling water" , "Great waterfalls on US-Canada border" , "N American river and Falls" .)