Serious player in the ring (11)
I believe the answer is:
heavyweight
'serious' is the definition.
Both the answer and definition are adjectives. Maybe there's an association between them I don't understand?
'player in the ring' is the wordplay.
I cannot really see how this works, but
'player' could be 'heavy' (heavy is a kind of player) and 'heavy' is found within the answer.
The remaining letters 'weight' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for heavyweight that I've seen before include "One like Goliath" , "Boxing class" , "Boxing category" , "Person of influence" , "Person of import - boxer" .)