Friend of Caesar, J? Just the opposite! (7)
I believe the answer is:
antonym
'just the opposite' is the definition.
(a word's antonym is its opposite)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for antonym that I've seen before include "As 'happy' is to 'sad' or 'long' to 'short'" , "A word's opposite" , "Nick's could be free" , "One's import is contrary" , "opposing term" .)