Dress shortly pregnant wife will wear (4)
I believe the answer is:
gown
'dress' is the definition.
(I know that gown is a type of dress)
'shortly pregnant wife will wear' is the wordplay.
'shortly' means to remove the last letter.
'pregnant' becomes 'gone' (eg 2 months gone means 2 months pregnant).
'wife' becomes 'w'.
'will wear' means one lot of letters goes inside another (letters on the outside are 'worn').
'gone' with its last letter taken off is 'gon'.
'gon' enclosing 'w' is 'GOWN'.
(Other definitions for gown that I've seen before include "Loose flowing garment for woman or judge" , "Elegant dress" , "Academic (or evening) wear" , "An evening dress" , "Dress for formal occasions" .)