Arthur's first minder worked with us — a female one (9)
I believe the answer is:
nursemaid
'a female one' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'arthur's first minder worked with us' is the wordplay.
'arthur's first' becomes 'a' (1st letter of 'arthur').
'worked with' indicates anagramming the letters.
'a'+'minder'='aminder'
'aminder' anagrammed with 'us' gives 'NURSEMAID'.
Can you help me to learn more?
(Other definitions for nursemaid that I've seen before include "Woman tending a child or children" , "Be overprotective towards (someone)" , "One looking after children" , "employ one for small charge" , "her charges are small" .)