Singular of staff

Solution 1:

The word staff used in this sense is already singular. It derives from the idea of support provided by a stick, a staff or stave.

It was originally a military term - for a staff of officers:

21. Mil. a. A body of officers appointed to assist a general, or other commanding officer, in the control of an army, brigade, regiment, etc., or in performing special duties (as the medical staff). General Staff, a body of officers controlling an army from headquarters under the commander-in-chief; hence Chief of the General Staff. Chief of Staff, the senior staff officer of a service or commander. [Apparently of continental Germanic origin. Compare the like use of German stab (also generalstab , regimentsstab , etc.), Dutch staf ; probably developed from the sense ‘baton’ (= 7
    above).]

But it has come to apply to any organisation which is managed:

22. a. gen. A body of persons employed, under the direction of a manager or chief, in the work of an establishment or the execution of some undertaking (e.g. a newspaper, hospital, government survey, school, etc.).

So I'm afraid the bad news is that since it is already singular, you cannot derive from it something which is "more singular". The only possibility, other than member of staff is to describe the person by name, e.g. the deputy assistant butler, twice removed. (Quotations from Oxford English Dictionary)

Solution 2:

There are some common words mentioned in the comments but there is a word derived from staff which is staffer. It is AmE but might not be that common. [The frequency is band 4 in OED]

OED defines as:

orig. and chiefly U.S.
A member of a staff.

a. Of a newspaper or journal: a staff writer.
b. More widely, of a business or other organization.
c. spec. of the President of the U.S.; a member of the President's White House staff.