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Indefatigable keeper? (6)

Ross

I believe the answer is:

stayer

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'indefatigable keeper?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

This is the entire clue.

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(Other definitions for stayer that I've seen before include "One with lasting qualities" , "one running over a long distance?" , "Racehorse perhaps" , "person hanging on" , "Person who lasts the course, doesn't quit" .)

I've seen this clue in the Evening Standard.
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