Crossword Genius

One partial to fish, Ruth has no starter (5)

Ross

I believe the answer is:

egret

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'one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for egret that I've seen before include "Lesser white heron" , "Type of heron" , "One of a pair of waders" , "white fish eater" , "Type of forty eight Ac" .)

I've seen this clue in The Sun.
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