Seriousness of natural attraction (7)
I believe the answer is:
gravity
This clue is a double definition.
'seriousness' is the first definition.
(gravity is a kind of seriousness)
'natural attraction' is the second definition.
(force which naturally attracts things)
'of' is the link.
(Other definitions for gravity that I've seen before include "Pulling power" , "cause producing mass movement" , "Seriousness - attractive force" , "Possible reason for dropping" , "Importance; a natural force" .)