The most common and most likely answer to this crossword clue is the 14 letter word SEPTUAGENARIAN
I've seen this crossword clue in The Times
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More clues leading to the result SEPTUAGENARIAN
Buddy explains!
The straight part
We know that "in her eighth decade?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in her eighth decade? = septuagenarian.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 SEPTUAGENARIAN mean?
SEPTUAGENARIAN noun- One who is between the age of 70 and 79, inclusive.
- Being between the age of 70 and 79, inclusive. In one's eighth decade.
- Of or relating to a septuagenarian.