The most common and most likely answer to this crossword clue is the 4 plus 3 letter word AGES AGO
I've seen this crossword clue in The Guardian
A
G
E
S
A
G
O
Buddy explains!
The straight part
We know that "A long time since/A long time since" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a long time since/a long time since = ages ago.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.