Artist when old retires south (5)
I believe the answer is:
degas
'artist' is the definition.
(I've seen this in other clues)
'old retires south' is the wordplay.
'old' becomes 'aged' (similar in meaning).
'retires' says the letters should be written in reverse.
'south' becomes 'S' (abbreviation).
'aged' backwards is 'dega'.
'dega'+'s'='DEGAS'
'when' acts as a link.
(Other definitions for degas that I've seen before include "Edgar ---, Fr. impressionist" , "Edgar -, Fr. sculptor/ painter" , "Edgar --, French painter and sculptor" , "Edgar -, French impressionist" , "Edgar -, Fr. artist" .)