Line in game’s jargon (5)
I believe the answer is:
lingo
'jargon' is the definition.
(I know that jargon can be written as lingo)
'line in game's' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'game' becomes 'go game' (go is a board game).
'l'+'in'+'go'='LINGO'
(Other definitions for lingo that I've seen before include "Foreign language or dialect: jargon" , "special words" , "speech" , "means to communicate" , "Jargon, foreign language (informal)" .)