Jack's predecessor (3)
I believe the answer is:
ike
'jack's predecessor' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ike that I've seen before include "President Eisenhower's nickname" , "US general who became president, in short" , "Nickname for Dwight Eisenhower" , "Nickname of old US president" , "Former US President (informally)" .)