Good breaking stone in game (5)
I believe the answer is:
rugby
'game' is the definition.
(rugby is a kind of game)
'good breaking stone' is the wordplay.
'good' becomes 'g' (abbreviation).
'breaking' means one lot of letters goes inside another.
'stone' becomes 'ruby' (I've seen this before).
'g' going inside 'ruby' is 'RUGBY'.
'in' acts as a link.
(Other definitions for rugby that I've seen before include "English city; game" , "Tom Brown's school" , "Midlands town" , "Fifteen-a-side sport -- Olympic event in 1900, 1908, 1920 and 1924" , "Game named after school" .)