So out, it's in (5)
I believe the answer is:
retro
'so out it's in' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for retro that I've seen before include "imitating past fashion" , "Imitative of a style from the recent past" , "Imitating a style from the recent past" , "bit of nostalgia" , "Fashion of yesteryear" .)