The most common and most likely answer to this crossword clue is the 15 letter word CROSS-REFERENCE
I've seen this crossword clue in The Times
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More clues leading to the result CROSS-REFERENCE
Buddy explains!
The straight part
We know that "Reminder of past content" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reminder of past content = cross-reference.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 CROSS-REFERENCE mean?
CROSS-REFERENCE noun- A reference or direction in one place in a book or other source of information to information at another place in the same work
- To provide something with a cross-reference.
- To compare two or more sources of information.