Falls in Arsenal's initial profit reversed by the Gunners (7)
I believe the answer is:
niagara
'falls' is the definition.
(Niagara Falls)
'arsenal's initial profit reversed by the gunners' is the wordplay.
'initial' indicates taking the first letters.
'profit' becomes 'gain' (synonyms).
'reversed' is a reversal indicator.
'by' means one lot of letters go next to another.
'the gunners' becomes 'RA' (Royal Artillery).
The first letter of 'arsenals' is 'a'.
'gain' back-to-front is 'niag'.
'a' put after 'niag' is 'niaga'.
'niaga'+'ra'='NIAGARA'
'in' acts as a link.
(Other definitions for niagara that I've seen before include "River flowing from Lake Erie into Lake Ontario" , "Famous waterfalls" , "& 22 Down North American cascade" , "it's quite a height" , "North American waterfall" .)