Companion learning about popular chemical element (8)
I believe the answer is:
chlorine
'chemical element' is the definition.
(I know that chlorine is a type of chemical element)
'companion learning about popular' is the wordplay.
'companion' becomes 'ch' (Companion of Honour).
'learning' becomes 'lore' (both can mean tradition or knowledge).
'about' indicates putting letters inside.
'popular' becomes 'in' ('in' can mean trendy).
'lore' going around 'in' is 'lorine'.
'ch'+'lorine'='CHLORINE'
(Other definitions for chlorine that I've seen before include "Atomic number 17" , "It's used to purify a swimming pool" , "Pale green gaseous element" , "Poison gas, with sodium makes salt" , "It's used to purify water" .)