Crossword Genius

One often seen out on the tiles? (6)

Ross

I believe the answer is:

roofer

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

I can't explain the rest of the clue.

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(Other definitions for roofer that I've seen before include "One works on the top of a house" , "High-level building worker" , "One putting top on building" , "eg Thatcher" , "Person working on top of a house" .)

I've seen this clue in The Sunday Times.
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