Crossword Genius

There's resistance to them by tough customers (5)

Ross

I believe the answer is:

sales

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

'customers' is the definition.
Although both the answer and definition are plural nouns, I don't understand how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for sales that I've seen before include "Periods when shops sell goods at discounted prices, bargain times" , "Periods when shops offer goods at reduced prices" , "commercial exchanges" , "Cheap offer periods" , "Occasions for buying at reduced prices" .)

I've seen this clue in the Evening Standard.
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