Seriousness that makes things fall (7)
I believe the answer is:
gravity
'seriousness' is the definition.
(gravity can mean seriousness or urgency)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for gravity that I've seen before include "Importance; a natural force" , "Pulling power" , "weight" , "Seriousness - attractive force" , "It causes objects to fall to the ground when dropped" .)