First couple deplored Jackeen's vulgarity; they've seen it before (4,2)
I believe the answer is:
deja vu
'first' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for deja vu that I've seen before include "Illusion of familiarity" , "Psychological phenomenon" , "Sense of history repeating" , "Unoriginal material" , "Already seen" .)