Model in pretty petticoat (4)
I believe the answer is:
type
'model' is the definition.
(thesaurus)
'in pretty petticoat' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TYPE' is hidden in the letters of 'pretty petticoat'.
(Other definitions for type that I've seen before include "Use a keyboard; sort" , "Class or group of a particular class" , "Category, sort" , "Kind or class as of people" , "Kind - kind of writing" .)