Half of army once more retreats and falls (7)

I believe the answer is:

niagara

I'm a little stuck... Click here to teach me more about this clue!

'falls' is the definition.
(Niagara Falls)

I don't understand the remainder of the clue.

(Other definitions for niagara that I've seen before include "A lot of falling water" , "River forming part of US-Canada border" , "North American waterfalls" , "Famous 12" , "Waterfall at Iran aga" .)

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app

Connecting...