As possibly to go round a planet (6)
I believe the answer is:
saturn
'a planet' is the definition.
(Saturn is an example)
'as possibly to go round' is the wordplay.
'possibly' indicates an anagram.
'to go round' becomes 'turn'.
'as' anagrammed gives 'sa'.
'sa'+'turn'='SATURN'
(Other definitions for saturn that I've seen before include "The dark planet" , "Titan its moon" , "Roman god - planet" , "Solar system planet" , "Sixth planet from the sun" .)