The idea that beef is about lb1 (6)
I believe the answer is:
belief
'the idea' is the definition.
(belief is a kind of idea)
'beef is about lb1' is the wordplay.
'is about' means one lot of letters goes inside another.
'lbl' becomes 'li' (I can't explain this - if you can you should give a lot more credence to this answer).
'beef' going around 'li' is 'BELIEF'.
'that' acts as a link.
Can you help me to learn more?
(Other definitions for belief that I've seen before include "Creed or opinion" , "Faith, credence" , "Firm conviction" , "Credo" , "Tenet, creed" .)