The idea that meat is about £1 (6)
I believe the answer is:
belief
'the idea' is the definition.
(belief is a kind of idea)
'meat is about ps1' is the wordplay.
'meat' becomes 'beef' (beef is a kind of meat).
'is about' indicates putting letters inside.
'psl' becomes 'li' (I can't justify this - if you can you should give a lot more credence to this answer).
'beef' placed around 'li' is 'BELIEF'.
'that' is the link.
Can you help me to learn more?
(Other definitions for belief that I've seen before include "Faith, credence" , "Credo" , "Religious conviction" , "Tenet, creed" , "Conviction, faith" .)