All leaders in opposition really antagonistic, Labour said (4)
I believe the answer is:
oral
'said' is the definition.
(both can mean spoken)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for oral that I've seen before include "Spoken exam" , "Exam using speech" , "Relating to speech" , "By word of mouth, say exam" , "Of the voice" .)