The idea that beef's about a pound (6)
I believe the answer is:
belief
'the idea' is the definition.
(belief is a kind of idea)
'beef's about a pound' is the wordplay.
'about' is an insertion indicator.
'a pound' becomes 'li' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'beef' enclosing '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 "Trust, faith (6)" , "Faith, credence" , "Principle accepted without proof" , "Conviction, faith" , "Tenet, creed" .)