A mother ran out – one in London race? (10)
I believe the answer is:
marathoner
'one in london race?' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'a mother ran out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'a'+'mother'+'ran'='amotherran'
'amotherran' with letters rearranged gives 'MARATHONER'.
Can you help me to learn more?
(Other definitions for marathoner that I've seen before include "runner" , "athlete" .)