Somebody who rambles for an hour about US president (5)
I believe the answer is:
hiker
'somebody who rambles' is the definition.
(a hiker goes rambling)
'an hour about us president' is the wordplay.
'an hour' becomes 'hr' ('hr' can be a synonym of 'hour').
'about' is an insertion indicator.
'us president' becomes 'ike' (nickname of Dwight Eisenhower).
'hr' placed around 'ike' is 'HIKER'.
'for' is the link.
(Other definitions for hiker that I've seen before include "Someone walking (with backpack?)" , "Person on long walk" , "Person on walkabout" , "Country walker" , "Long-distance walker" .)