Seriousness will tend to keep you grounded (7)
I believe the answer is:
gravity
I believe this clue is a double definition.
'seriousness' is the first definition.
(gravity can mean seriousness or urgency)
'will tend to keep you grounded' is the second definition.
(gravity keeps you on the ground)
(Other definitions for gravity that I've seen before include "Seriousness - attractive force" , "The force of attraction between masses in the universe" , "weight" , "Importance; a natural force" , "It causes objects to fall to the ground when dropped" .)