Lines going around the world will snub Luther's latest theses (6)
I believe the answer is:
topics
'snub luther's latest theses' is the definition.
I can't tell whether this defines the answer.
'lines going around the world will' is the wordplay.
'lines' becomes 'tops' (top is a kind of line).
'going around' indicates putting letters inside.
'the world will' becomes 'ic' (I can't explain this - if you can you should give a lot more credence to this answer).
'tops' placed around 'ic' is 'TOPICS'.
(Other definitions for topics that I've seen before include "'Subjects, issues (6)'" , "Subjects to talk about" , "Subjects about optics" , "Subjects of discussions" , "Subjects under discussion" .)