Crossword Genius

Nile-dweller's pupils in formation? (9)

Ross

I believe the answer is:

crocodile

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'formation?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for crocodile that I've seen before include "Large river reptile" , "Old Cicero, oddly, was a voracious reptile" , "Vicious beast" , "Predatory reptile" , "Line of children walking in twos" .)

I've seen this clue in The Guardian.
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