The most common and most likely answer to this crossword clue is the 7 letter word TONTINE
I've seen this crossword clue in The Guardian
T
O
N
T
I
N
E
Buddy explains!

The straight part
We know that "annuity scheme/annuity scheme" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case annuity scheme/annuity scheme = tontine.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.
What does TONTINE mean?
TONTINE noun- A form of investment in which, on the death of an investor, his share is divided amongst the other investors.