Beaten by a single stroke (4)
I believe the answer is:
aced
'beaten' is the definition.
The answer and definition can be both related to competing as well as being past participle verbs.
Maybe there's an association between them I don't understand?
'by a single stroke' is the wordplay.
'by' is a charade indicator (letters next to each other).
'a single' becomes 'a'.
'stroke' becomes 'ced' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'ced' is 'ACED'.
Can you help me to learn more?
(Other definitions for aced that I've seen before include "passed with highest marks in US" , "Comprehensively beaten" , "Sailed through" , "got a hole in one" , "Reached a golf hole in one shot" .)