When retired go round world (6)
I believe the answer is:
saturn
'world' is the definition.
(Saturn is another planet)
'when retired go round' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'retired' shows that the letters should be reversed in order.
'go round' becomes 'turn' (both can mean to rotate).
'as' backwards is 'sa'.
'sa'+'turn'='SATURN'
(Other definitions for saturn that I've seen before include "The dark planet" , "Massive body ringed" , "One of the 1 across" , "Roman god - gas giant" , "Roman god - planet" .)