The most common and most likely answer to this crossword clue is the 10 plus 5 letter word SUSPICIOUS MINDS
I've seen this crossword clue in Financial Times
S
U
S
P
I
C
I
O
U
S
M
I
N
D
S
Buddy explains!
The straight part
We know that "number" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case number = suspicious minds.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.