@Miss L Toad,
Oh I say, you are a brick. Doubtless the old grey matter has been working overtime as has mine (well, what's left of it).
Here at home I have checked two dictionaries of literary terms as well as Roget's thesaurus and the Oxford thesaurus. Plus Fowler's Modern English Usage and all the relevant sites I can find on line. Result: zilch.
I've also tried searching for words beginning and ending with the same three letters in the same order. Although there are quite a lot listed, none meets Bill's definition and none relates to grammatical expression.
And yet, it seems that several GFQ competitors have had no trouble finding the answer. I had considered submitting "oxymoronoxy" as my answer but the word simply doesn't exist. I really am puzzled by this one.