A planet, by Jove! (7)
I believe the answer is:
jupiter
This clue is a double definition.
'a planet' is the first definition.
(Jupiter is an example)
'jove' is the second definition.
(I know that jove can be written as Jupiter)
'by' acts as a 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" .)