Low attack involving Northern Line (8)
I believe the answer is:
monorail
'line' is the definition.
(type of railway line)
'low attack involving northern' is the wordplay.
'low' becomes 'moo' (both can mean to make a cow noise).
'attack' becomes 'rail' (I can't explain this - if you can you should give a lot more credence to this answer).
'involving' is an insertion indicator.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'moo'+'rail'='moorail'
'moorail' enclosing 'n' is 'MONORAIL'.
Can you help me to learn more?
(Other definitions for monorail that I've seen before include "Suspended railway" , "Hanging transport system" , "Form of transport" , "train" , "Type of railway" .)