Wilson's predecessor (4)
I believe the answer is:
taft
'wilson's predecessor' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for taft that I've seen before include "US president, 1909-13" , "27th US president, William" , "US President before Wilson" , "former President" , "27th President of the United States" .)