Word meaning the opposite (7)
I believe the answer is:
antonym
'word meaning the opposite' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for antonym that I've seen before include "as host is for guest, eg" , "something completely different" , "opposing term" , "As 'happy' is to 'sad' or 'long' to 'short'" , "A word opposite in meaning to another" .)