Conviction behind complaint about Lithium (6)
I believe the answer is:
belief
'conviction' is the definition.
(I've seen this in another clue)
'behind complaint about lithium' is the wordplay.
'behind complaint' becomes 'beef' (beef can mean a complaint or objection. I am not sure about the 'behind' bit.).
'about' is an insertion indicator.
'lithium' becomes 'Li' (chemical symbol).
'beef' enclosing 'li' is 'BELIEF'.
Can you help me to learn more?
(Other definitions for belief that I've seen before include "Trust, faith (6)" , "Conviction, faith" , "Religious conviction" , "Principle accepted without proof" , "Credo" .)