The most common and most likely answer to this crossword clue is the 11 letter word MEMORABILIA
M
E
M
O
R
A
B
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A
Buddy explains!
The straight part
We know that "evidence of past activity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case evidence of past activity = memorabilia.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 MEMORABILIA mean?
MEMORABILIA noun- Objects that are connected to or remind their owner of past events.
- Things worth remembering: noteworthy points.