Infant's outfit (7)
(Other definitions for layette that I've seen before include "Kit for baby" , "Baby clothes and accessories" , "Provision for new baby" , "Outfit for a new baby" , "First outfit" .)
'infant's outfit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for onesie that I've seen before include "One-piece full-body garment" , "Single-piece full body garment" .)