Sure Ray changed the dividing line (8)
I believe the answer is:
boundary
'dividing line' is the definition.
'boundary' can be an answer for 'line' (boundary is a kind of line). I am unsure of the 'dividing' bit.
'sure ray changed' is the wordplay.
'sure' becomes 'bound' (synonyms).
'changed' indicates anagramming the letters.
'ray' is an anagram of 'ary'.
'bound'+'ary'='BOUNDARY'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for boundary that I've seen before include "Line marking the outer limits" , "Limit (of an area)" , "Perimeter: demarcation" , "line between states" , "Four, perhaps" .)