Go in (5)
'go in' is the definition.
(as in entering a building)
This is all the clue.
(Other definitions for enter that I've seen before include "A key on the right" , "Treen (anag.)" , "Go/come in" , "cross the threshold" , "submit?" .)
(Other definitions for offer that I've seen before include "Chance to buy at reduced price?" , "Submit" , "killer?" , "Tender for acceptance" , "Propose; make available" .)