Crossword Genius

Handy source of opposition? (5)

Ross

I believe the answer is:

thumb

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This clue might be a double definition.

'handy source' is the first definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

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

'of' is the link.

Can you help me to learn more?

(Other definitions for thumb that I've seen before include "Pollex, in humans" , "Try to get free ride" , "Quickly turn pages" , "Tom's digital fairy tale" , "Second part of 12" .)

I've seen this clue in The Times.
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