Falls once more over trap's middle (7)
I believe the answer is:
niagara
'falls' is the definition.
(Niagara Falls)
'once more over trap's middle' is the wordplay.
'once more' becomes 'again' (synonyms).
'over' says the letters should be written in reverse.
'middle' says to take the centre.
The middle letters of 'trap' are 'ra'.
'again' backwards is 'niaga'.
'niaga'+'ra'='NIAGARA'
(Other definitions for niagara that I've seen before include "They fall in North America" , "Great falls on border of Canada and USA" , "1953 Marilyn Monroe film noir thriller" , "North American waterfall" , "N. American waterfall" .)