The most common and most likely answer to this crossword clue is the 4 plus 5 letter word LATE RISER
I've seen this crossword clue in The Guardian
L
A
T
E
R
I
S
E
R
Buddy explains!
The straight part
We know that "rarely see one when they open" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rarely see one when they open = late riser.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.