No but there is a FIRST (version 1.5) function which could give you the same
result as last in certain circumstances.

The first function allows you to return only the first n rows of the results
of a given select query.

Let others correct me if I'm incorrect or if my syntax is off.
To give you exactly what you want by a stored procedure you would:

CREATE PROCEDURE TOTAL_LAST_CLIENTS
RETURNS (ANNO INTEGER /*OR WHATEVER TYPE YOU ARE USING*/),
CLIENT INTEGER /OR *WHATEVER YOU TYPE YOU ARE USING*/,
TOTAL DECIMAL(18,6))
AS
BEGIN
FOR SELECT DISTINCT ANNO FROM PAYMENT ORDER BY ANNO INTO :ANNO DO
BEGIN
SELECT FIRST 1 CLIENT FROM PAYMENT
WHERE (ANNO = :ANNO)
ORDER BY CLIENT DESC
INTO :CLIENT
SELECT SUM(IMPORT) FROM PAYMENT
WHERE (ANNO = :ANNO) AND (CLIENT = :CLIENT)
INTO :TOTAL
SUSPEND
END
END

I'm sure it would be possible to do the same thing outside of a stored
procedure using nested sql if you have the time to work it out.
IMO, It will be somewhat more complicated than the stored procedure.

regards,
Kevin Day.