Asian runner in French south retired (5)
I believe the answer is:
indus
'asian runner' is the definition.
(I've seen this before)
'in french south retired' is the wordplay.
'french south' becomes 'sud' (I am not sure about this - if you are sure you should believe this answer much more).
'retired' says the letters should be written backwards.
'sud' back-to-front is 'dus'.
'in'+'dus'='INDUS'
Can you help me to learn more?
(Other definitions for indus that I've seen before include "running water" , "Main river of Pakistan" , "Pakistani river" , "River of India and Pakistan" , "River of Southern Asia" .)