By a thumping good score? (6)
I believe the answer is:
beaten
'a thumping good score?' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'by' is the wordplay.
I cannot really see how this works, but
'by' could be 'at' and 'at' is found within the answer.
The remaining letters 'been' 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 beaten that I've seen before include "Stuffed" , "Well used" , "Formed or made thin by hammering" , "Defeated or hit repeatedly" , "Conquered" .)