Just love Northern Line railway in the end (4)
I believe the answer is:
only
'just' is the definition.
('only' can be a synonym of 'just')
'love northern line railway in the end' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'line' becomes 'l'.
'in the end' suggests the final letters.
The last letter of 'railway' is 'y'.
'o'+'n'+'l'+'y'='ONLY'
(Other definitions for only that I've seen before include "Unique; merely" , "Single, solitary" , "Solely, lone" , "Merely; single" , "See 6" .)