While Lavatch was referring us around the various dictionaries, I’m surprised no mention was made of the fact that the spelling of 7 down, specifically its final letter, is not to be found in Chambers - well, not my 11th edition anyway - but it turns up in COD and ODE. Either that or my answer’s wrong!
According to my Oxford Dictionary of English, picked up for £5 last year in a sale at my local WH Smith and now in my lap as I type, the word is tsessebi, which also fits the clue's word-play.