Cryptic crossword clue
Momentous exhibition’s first opening, almost filled to capacity 8

The most common and most likely answer to this crossword clue is the 8 letter word EVENTFUL

I've seen this crossword clue in The Guardian
E
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Buddy explains!

The straight part
We know that "Momentous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case momentous = eventful.
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 EVENTFUL mean?

EVENTFUL adjective
  1. Of or pertaining to high levels of activity; having many memorable events.

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