Apron (5)
(Other definitions for pinny that I've seen before include "Apron (abbr)" , "Apron (abbrev.)" , "Apron-like protective garment (abbreviated)" .)
'apron' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pinafore that I've seen before include "G&S ship" , "Sort of apron" , "Stagecraft, say" , "HMS -- (G&S)" , "'HMS - - ' G & S" .)