Back in (5)
I believe the answer is:
retro
'back in' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for retro that I've seen before include "Looking back" , "Reliving the past" , "recapturing the past" , "Imitating a style from the recent past" , "in nostalgic style" .)