Big fall in Canadian and US currency (7)

I believe the answer is:

niagara

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

'big fall' is the definition.
'Niagara' can be an answer for 'fall' (thesaurus). I'm not certain of the 'big' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for niagara that I've seen before include "Large water falls" , "They fall in North America" , "Great waterfalls on US-Canada border" , "Famous waterfall" , "North American waterfall" .)

I've seen this clue in The Globe and Mail.

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