Reg and Sadie unfortunately don't see eye to eye (8)
I believe the answer is:
disagree
'don't see eye to eye' is the definition.
I can't judge whether this defines the answer.
'reg and sadie unfortunately' is the wordplay.
'and' means one lot of letters go next to another.
'unfortunately' indicates an anagram (a bad or unfortunate spelling).
'reg' put after 'sadie' is 'sadiereg'.
'sadiereg' is an anagram of 'DISAGREE'.
Can you help me to learn more?
(Other definitions for disagree that I've seen before include "Have words" , "Dire ages (anag.)" , "Take a different view" , "Differ in opinion" , "Hold different opinions" .)