Alarmingly, boxer hit international expo participant (9)
I believe the answer is:
exhibitor
'participant' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for exhibitor that I've seen before include "shower" , "I hit boxer (anag.)" , "showman" , "Person displaying" , "One displaying (at show)" .)