Girl, middle-class, over nine maybe (5)
I believe the answer is:
annie
'girl' is the definition.
'middle-class over nine maybe' is the wordplay.
'middle' indicates the central letters.
'over' says to put letters next to each other.
'maybe' is an anagram indicator.
The central letter of 'class' is 'a'.
'nine' anagrammed gives 'nnie'.
'a'+'nnie'='ANNIE'
(Other definitions for annie that I've seen before include "Oakley, Laurie or Hall" , "Miss Laurie" , "Musical about red-handed Orphan girl" , "said to carry a gun" , "Musical about markswoman" .)