River once more receding with rain reduced by half (7)
I believe the answer is:
niagara
'river' is the definition.
(thesaurus)
'once more receding with rain reduced by half' is the wordplay.
'once more' becomes 'again' ('again' can be a synonym of 'once more'**).
'receding' says the letters should be written in reverse.
'with' is a charade indicator (letters next to each other).
'reduced by half' suggests halving the letters.
'again' reversed gives 'niaga'.
'rain' with half the letters taken is 'ra'.
'niaga'+'ra'='NIAGARA'
(Other definitions for niagara that I've seen before include "River linking Lakes Ontario and Erie" , "Horseshoe, American and Bridal Veil Falls, collectively" , "It falls in North America" , "US/Canadian falls" , "it's quite a height" .)