Sister in cotton uniform (3)
I believe the answer is:
nun
'sister' is the definition.
(thesaurus)
'in cotton uniform' is the wordplay.
'in' says the answer is hidden in the clue.
'NUN' is hidden in the letters of 'cotton uniform'.
(Other definitions for nun that I've seen before include "Member of convent" , "Joshua's father" , "Woman respecting order" , "Convent resident" , "Mother Superior, perhaps" .)