Falls back again with gunners (7)
I believe the answer is:
niagara
'falls' is the definition.
(I've seen this before)
'back again with gunners' is the wordplay.
'back' says the letters should be written backwards.
'with' means one lot of letters go next to another.
'gunners' becomes 'ra' (Royal Artillery).
'again' in reverse letter order is 'niaga'.
'niaga'+'ra'='NIAGARA'
(Other definitions for niagara that I've seen before include "River flowing from Lake Erie into Lake Ontario" , "N. American waterfall" , "US/Canadian falls" , "Famous waterfall" , "North American river with famous falls" .)