Girl gets on about noon (5)
I believe the answer is:
agnes
'girl' is the definition.
'gets on about noon' is the wordplay.
'gets on' becomes 'ages' ('age' can be a synonym of 'get on'**).
'about' is an insertion indicator.
'noon' becomes 'n'.
'ages' enclosing 'n' is 'AGNES'.
(Other definitions for agnes that I've seen before include "Patron saint of virgins" , "St --' Eve, poem by Lord Tennyson" , "girl" , "Miss Grey" , "lass" .)