Falls once more, going west around centre of Carmarthen (7)
I believe the answer is:
niagara
'falls' is the definition.
(Niagara Falls)
'once more going west around centre of carmarthen' is the wordplay.
'once more' becomes 'again' (synonyms).
'going west' shows that the letters should be reversed in order (letters heading to the west or left).
'around' shows that the letters should be reversed in order.
'centre of' means to look at the middle letters.
The middle of 'carmarthen' is 'ar'.
'again' back-to-front is 'niaga'.
'ar' in reverse letter order is 'ra'.
'niaga'+'ra'='NIAGARA'
(Other definitions for niagara that I've seen before include "& 22 Down North American cascade" , "A lot of falling water" , "Great waterfalls on Canadian-US border" , "Famous 12" , "Horseshoe, American and Bridal Veil Falls, collectively" .)