Portal with bar up, zero getting in (4)
I believe the answer is:
door
'portal' is the definition.
(synonyms)
'bar up zero getting in' is the wordplay.
'bar' becomes 'rod' (both can mean a pole or stick).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'zero' becomes 'O' (O resembles 0).
'getting in' means one lot of letters goes inside another.
'rod' in reverse letter order is 'dor'.
'dor' placed around 'o' is 'DOOR'.
'with' is the link.
(Other definitions for door that I've seen before include "Barrier to open and close" , "Means of exit and entrance" , "Entrance way" , "Opener" , "Portal" .)