Low bar spans Northern Line (8)
I believe the answer is:
monorail
'line' is the definition.
(type of railway line)
'low bar spans northern' is the wordplay.
'low' becomes 'moo' (both can mean to make the noise of a cow).
'bar' becomes 'rail' (rail is a kind of bar).
'spans' is an insertion indicator (span can mean to cross or go over).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'moo'+'rail'='moorail'
'moorail' enclosing 'n' is 'MONORAIL'.
(Other definitions for monorail that I've seen before include "Hanging transport system" , "Suspended railway" , "train" , "Type of railway" , "Form of transport" .)