Low resistance up in space (4)
I believe the answer is:
room
'space' is the definition.
(room is a kind of space)
'low resistance up' is the wordplay.
'low' becomes 'moo' ('moo' can be a synonym of 'low').
'resistance' becomes 'r' (abbreviation).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'moo'+'r'='moor'
'moor' back-to-front is 'ROOM'.
'in' is the link.
(Other definitions for room that I've seen before include "Enclosed indoor space" , "Enclosed space in a building" , "Box ---, storage area" , "bedsit?" , "Chamber; scope" .)