The most common and most likely answer to this crossword clue is the 9 letter word OVERNIGHT
I've seen this crossword clue in The Guardian
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More clues leading to the result OVERNIGHT
- Top man at Camelot, we hear, throughout a dark period?
- Sir losing head over maiden, maybe surprisingly quickly
- Maiden possibly meeting horrible thing during dark period
- Plain sandwiches almost lasting until tomorrow
- Open and close inside short period
- Very quickly sir loses head when meeting maiden?
- Very swiftly finished – nearly on time
- Voting her out within a very short time
- Her voting is erratic during the hours of darkness
- Completed complex thing suddenly
Buddy explains!

The straight part
We know that "short period" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case short period = overnight.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 OVERNIGHT mean?
OVERNIGHT noun- Items delivered or completed overnight.
- An overnight stay, especially in a hotel or other lodging facility.
- (in the plural) Viewership ratings for a television show that are published the morning after it is broadcast, and may be revised later on.
- The fore part of the previous night; yesterday evening.
- To stay overnight; to spend the night.
- To send something for delivery the next day.
- Occurring between dusk and dawn.
- Complete before the next morning.
- Of an activity or event in which participants stay overnight.
- During or throughout the night, especially during the evening or night just past.
- In a very short (but unspecified) amount of time.