The most common and most likely answer to this crossword clue is the 5 letter word UNZIP
I've seen this crossword clue in Financial Times
U
N
Z
I
P
More clues leading to the result UNZIP
Buddy explains!

The straight part
We know that "open" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case open = unzip.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 UNZIP mean?
UNZIP verb- To open something using a zipper.
- To come open by means of a zipper.
- To decompress (a zip file).