The most common and most likely answer to this crossword clue is the 8 plus 6 letter word UNEARNED INCOME
I've seen this crossword clue in Independent
U
N
E
A
R
N
E
D
I
N
C
O
M
E
Buddy explains!
The straight part
We know that "interest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case interest = unearned income.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.