Talk about line on square vessel (5)
I believe the answer is:
glass
'vessel' is the definition.
(thesaurus)
'talk about line on square' is the wordplay.
'talk' becomes 'gas' (gas can mean to chat).
'about' is an insertion indicator.
'line on' becomes 'l' (used when specifying particular lines from a poem. I am not sure about the 'on' bit.).
'square' becomes 's' (abbreviation).
'gas' placed around 'l' is 'glas'.
'glas'+'s'='GLASS'
Can you help me to learn more?
(Other definitions for glass that I've seen before include "Brittle transparent material" , "Tumbler, mirror" , "Something to drink from" , "Telescope perhaps" , "Translucent, usually transparent, material" .)