Nothing divides the lines – that's the idea (6)
I believe the answer is:
theory
'the idea' is the definition.
(theory is a kind of idea)
'nothing divides the lines' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'divides' means one lot of letters goes inside another.
'lines' becomes 'ry' (abbreviation for railway).
'the'+'ry'='thery'
'o' going into 'thery' is 'THEORY'.
'that's' is the link.
(Other definitions for theory that I've seen before include "Ideal situation" , "Conjecture, hypothesis" , "Speculative view" , "Idea for testing" , "General principles of a subject" .)