The most common and most likely answer to this crossword clue is the 8 plus 7 letter word BUILDING SOCIETY
I've seen this crossword clue in Financial Times
B
U
I
L
D
I
N
G
S
O
C
I
E
T
Y
Buddy explains!
The straight part
We know that "Co-op financier" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case co-op financier = building society.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.