The most common and most likely answer to this crossword clue is the 4 plus 9 letter word CASE SENSITIVE
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The cryptic/wordplay part
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What does CASE SENSITIVE mean?
CASE SENSITIVE adjective- Distinguishing upper- and lower-case letters. Often used in computer science to indicate a distinction is made in comparison or equality of letters based on case. For example, a case-sensitive password will not recognize "Password" and "password" as the same, but a case insensitive comparison would.