I am one of the researchers who contribute to Acuff.org. You will want to look at that site, if you haven't done that already. (It is all free, but you have to register to see the Family Tree.)
We think you are right that Clara is a daughter of William S. and Mary (Talley) Acuff. One of her nieces told me that she had heard that Fannie Clara Acuff had a nervous breakdown after being raped and was sent to the mental hospital.
I am not sure exactly when William and Mary were divorced, but Mary remarried before 1910. In the 1910 census, Clara and her 5 living siblings were listed with her mother and stepfather -- and their surname was misspelled as ACKUS or ACKERS.
1910 federal census of Fort Myers, Lee County, Florida
Roll: T624_163; sheet 12A; Enumeration District: 79, Prec. 4, page 140A
John A. Mayfield Head MW 48 married for the 2nd time, 5 years TN-TN-TN House carpenter. Literate.
Mary M. Mayfield Wife FW 41 married for the 2nd time, 5 years 7 children, 6 living MO-TN-TN. Spaces for indicating literacy are blank.
Elbert F. Ackus stepson MW 21 single TX-TN-MO. Fisherman, general, on his own account. Literate.
Roy Ackus stepson MW 16 single TX-TN-MO. Captain, launch, on his own account. Literate.
Rupert Ackus stepson MW 13 single TX-TN-MO. Literate; attending school.
Eustace Ackus stepson MW 11 single TX-TN-MO. Literate; attending school.
Arthur Ackus stepson MW 9 [? - overwritten; could be 7 or 8] single FL-TN-MO Not marked as literate or attending school
Clara [sic] F. Ackus stepdaughter FW 17 single TX-TN-MO. Literate
Clara is listed last, out of age order, which probably means that she was living in the hospital then, as the census entry you found shows.