40th US President (6)
I believe the answer is:
reagan
'40th us president' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for reagan that I've seen before include "40th US president, d. 2004" , "US president, 1981-89" , "Former leader" , "Ronald ...... ex-American president" , "Ronald . . . . . ., 40th president of USA" .)