The most common and most likely answer to this crossword clue is the 7 letter word BECAUSE
I've seen this crossword clue in Independent
B
E
C
A
U
S
E
Buddy explains!

The straight part
We know that "Since" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case since = because.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 BECAUSE mean?
BECAUSE adverb- For the reason (that).
- On account (of), for sake (of).
- (by ellipsis) Used alone to refuse to provide a full answer a question begun with "why", usually taken as an anapodoton of the elided full phrase "Because I said so".
- On account of, because of.
- By or for the cause that; on this account that; for the reason that.
- As is known, inferred, or determined from the fact that.
- So that, in order that.