The most common and most likely answer to this crossword clue is the 10 plus 5 letter word GOVERNMENT BONDS
I've seen this crossword clue in The Daily Telegraph
G
O
V
E
R
N
M
E
N
T
B
O
N
D
S
Buddy explains!
The straight part
We know that "financial certificates" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case financial certificates = government bonds.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.