Girl, one with two names (3)
I believe the answer is:
ann
'girl' is the definition.
'one with two names' is the wordplay.
'two names' means two substitutions for 'name'.
'one with' is an insertion indicator.
'name' becomes 'n' (common abbreviation e.g. on forms).
'name' becomes 'an' (I am not sure about this - if you are sure you should believe this answer much more).
'n' going within 'an' is 'ANN'.
Can you help me to learn more?
(Other definitions for ann that I've seen before include "Ms Widdecombe" , "Girl's name" , "... Margret or Hathaway" , "eg Miss Widdecombe" , "Girl" .)