Go back in (7)
'go back in' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for re-enter that I've seen before include "compete again" , "Come or go in again" , "Come back inside" , "Go back in" .)
(Other definitions for reenter that I've seen before include "Terrene (anag.)" , "Come back inside" , "Go in again" , "Penetrate again" , "Go/come in again" .)