Long-distance runner from Basel inevitably retired (4)
I believe the answer is:
nile
'long-distance runner' is the definition.
(long river - something that runs)
'from basel inevitably retired' is the wordplay.
'from' indicates a hidden word.
'retired' says the letters should be written backwards.
'elin' is hidden in the letters of 'basel inevitably'.
'elin' back-to-front is 'NILE'.
(Other definitions for nile that I've seen before include "African river" , "River associated with Cleopatra" , "Take a line to the river" , "Green" , "World's longest river" .)