In such activity bell sounds — seconds out, round eleven (6)
I believe the answer is:
boxing
'in such activity bell' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'sounds seconds out round eleven' is the wordplay.
'sounds seconds out' becomes 'bong' (I can't explain this - if you can you should give a lot more credence to this answer).
'round' indicates putting letters inside.
'eleven' becomes 'xi' ('XI' can be a synonym of 'eleven').
'bong' going around 'xi' is 'BOXING'.
(Other definitions for boxing that I've seen before include "Fighting with the fists" , "__ Day; December 26" , "Packer's job" , "Fighting in a ring" , "Ring sport" .)