The girl’s originally employed in personnel (3)
I believe the answer is:
her
'the girl's' is the definition.
'originally employed in personnel' is the wordplay.
'originally' says to take the initial letters.
'in' indicates putting letters inside.
'personnel' becomes 'hr' (Human Resources).
The first letter of 'employed' is 'e'.
'e' placed inside 'hr' is 'HER'.
(Other definitions for her that I've seen before include "Pronoun used of ship" , "The female's" , "In the Midlands she" , "That lady's" , "That lady" .)