The most common and most likely answer to this crossword clue is the 6 plus 4 letter word FLOPPY DISK
I've seen this crossword clue in The Guardian
F
L
O
P
P
Y
D
I
S
K
More clues leading to the result FLOPPY DISK
Buddy explains!
The straight part
We know that "an old retainer for information/an old retainer for information" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an old retainer for information/an old retainer for information = floppy disk.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.