Being doubly good is simply great (3)
I believe the answer is:
big
'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" .)