Girl, to an Aussie (6)
I believe the answer is:
'girl to an aussie' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sheila that I've seen before include "Female name" , "Girl's name (any Oz girl?)" , "Girl's name; Six down (anag.)" , "Woman (Aus. colloq.)" , "Informal Australian girl - could be Irish" .)