The most common and most likely answer to this crossword clue is the 8 letter word LOCKDOWN
I've seen this crossword clue in Independent
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More clues leading to the result LOCKDOWN
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The straight part
We know that "security measure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case security measure = lockdown.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 LOCKDOWN mean?
LOCKDOWN noun- The confinement of people in their own rooms (e.g. in a school) or cells (in a prison), or to their own homes or areas (e.g. in the case of a city- or nation-wide issue) as a security measure after or amid a disturbance or pandemic, etc.
- A contrivance to fasten logs together in rafting.