Sensible planet's responsibility? (4-2-5)

I believe the answer is:

down-to-earth

I'm a little stuck... Click here to teach me more about this clue!

'sensible' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for down-to-earth that I've seen before include "Far from assuming" , "Sensible - practical - realistic" , "where Newton's apple fell?" , "Practical, no-nonsense" .)

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app