Struggle to keep within ring that’s on a slant (8)
I believe the answer is:
diagonal
'on a slant' is the definition.
'struggle to keep within ring' is the wordplay.
'struggle' becomes 'agon' (ancient Greek term for conflict).
'to keep within' is an insertion indicator.
'ring' becomes 'dial' (I've seen this before).
'agon' placed into 'dial' is 'DIAGONAL'.
'that's' acts as a link.
(Other definitions for diagonal that I've seen before include "bishop moves here" , "straight line from corner" , "It joins square's opposite corners" , "bishop controls it" , "Line from one corner to the fartherst one" .)