Falls back once more having run ahead initially (7)
I believe the answer is:
niagara
'falls' is the definition.
(thesaurus)
'back once more having run ahead initially' is the wordplay.
'back' is a reversal indicator.
'once more' becomes 'again' ('again' can be a synonym of 'once more'**).
'having' means one lot of letters go next to another.
'initially' suggests taking the first letters.
The initial letters of 'run ahead' is 'ra'.
'again' written backwards gives 'niaga'.
'niaga'+'ra'='NIAGARA'
(Other definitions for niagara that I've seen before include "North American waterfall" , "River linking Lakes Ontario and Erie" , "Horseshoe, American and Bridal Veil Falls, collectively" , "Famous waterfall" , "Great falls on border of Canada and USA" .)