The most common and most likely answer to this crossword clue is the 15 letter word ANTEPENULTIMATE
I've seen this crossword clue in The Sunday Telegraph
A
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More clues leading to the result ANTEPENULTIMATE
Buddy explains!
The straight part
We know that "manipulate tenet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case manipulate tenet = antepenultimate.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ANTEPENULTIMATE mean?
ANTEPENULTIMATE noun- Two before the last in a series. e.g. (..., antepenultimate, penultimate, ultimate)
- The syllable that comes two before the last in a word.
- Two before the last, i.e., the one immediately before the penultimate, in a series.