At his sport, he's lethal! (7)
I believe the answer is:
matador
'at his sport he's lethal' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'at his sport he's' is the wordplay.
I cannot quite understand how this works, but
'at' is present in the answer.
'he' could be 'ado' and 'ado' is found in the answer.
The remaining letters 'mr' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for matador that I've seen before include "A bull-fighter.11 Beginning to develop" , "The one who kills the bull" , "he aims to get a bull" , "A Spanish bullfighter" , "He kills the bull in formal fight" .)