The most common and most likely answer to this crossword clue is the 5 plus 2 letter word CHECK IN
I've seen this crossword clue in The Guardian
C
H
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More clues leading to the result CHECK IN
Buddy explains!
The straight part
We know that "register" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case register = check in.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 CHECK IN mean?
CHECK IN noun- The act of checking in at a hotel, airport, etc.
- The submission of source code or other material to a source control repository.