Crossword Genius

Being doubly good is simply great (3)

Ross

I believe the answer is:

big

I'm a little stuck... Click here to teach me more about this clue!

'great' is the definition.
(I know that great can be written as big)

'being doubly good is simply' is the wordplay.
I cannot really understand how this works, but
'good' could be 'g' (abbreviation) and 'g' is found in the answer.
The remaining letters 'bi' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for big that I've seen before include "Large; generous" , "London bell" , "Large or substantial" , "Circus tent" , "Huge" .)

I've seen this clue in the Evening Standard.
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