A planet, by Jove! (7)
I believe the answer is:
jupiter
I believe this clue is a double definition.
'a planet' is the first definition.
(Jupiter is an example)
'jove' is the second definition.
(synonyms)
'by' is the link.
(Other definitions for jupiter that I've seen before include "Largest planet in our solar system" , "Fifth planet from sun" , "Planet - Roman equivalent of Zeus" , "Chief Roman god" , "Large planet" .)