Girl's taking position as cashier (8)
I believe the answer is:
displace
'cashier' is the definition.
(thesaurus)
'girl's taking position' is the wordplay.
'girl's' becomes 'dis'.
'taking' says to put letters next to each other.
'position' becomes 'place' ('place' can be a synonym of 'position'**).
'dis'+'place'='DISPLACE'
'as' acts as a link.
(Other definitions for displace that I've seen before include "Succeed" , "Remove from usual habitat" , "Supplant" , "Move out of position" , "Shift" .)