Lyndon Johnson successor (5)
I believe the answer is:
nixon
'lyndon johnson successor' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nixon that I've seen before include "US president, d.1994" , "Watergate US president" , "US president before Gerald Ford" , "President re-elected in 1972" , "Only US president to have resigned from office" .)