former US president (5)
'former us president' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nixon that I've seen before include "Ex-US president" , "37th president of the United States, resigned 1974" , "Politician" , "American president known as tricky Dicky" , "President re-elected in 1972" .)
(Other definitions for obama that I've seen before include "US president born in Hawaii" , "one with top job in Washington" , "44th US President" , "Important figure" , "Former president" .)