The immortal Mr Heyerdahl (4)
I believe the answer is:
thor
This clue is a triple definition!
'the immortal' is the first definition.
(Thor is an example)
'mr' is the second definition.
'heyerdahl' is the third definition.
(I've seen this in another clue)
(Other definitions for thor that I've seen before include "Scandinavian thunder-god" , "Norse deity" , "One of the Norse gods" , "Wielder of hammer" , "Norse hammer-wielding god" .)