The most common and most likely answer to this crossword clue is the 1 plus 5 plus 3 letter word A RAINY DAY
I've seen this crossword clue in The Times
A
R
A
I
N
Y
D
A
Y
Buddy explains!
The straight part
We know that "save for this" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case save for this = a rainy day.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.