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Relatively speaking the greater part of an eruption (5)

Ross

I believe the answer is:

uncle

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

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

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(Other definitions for uncle that I've seen before include "Family relation" , "Male family member" , "Slang name for pawnbroker" , "Pawnbroker's nickname" , "one who's taken the pledge" .)

I've seen this clue in The Globe and Mail.
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