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 can't justify this - if you can you should believe this answer much more).
'beef' enclosing 'li' is 'BELIEF'.
'that' is the 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" .)