The importance of gravity (6)
I believe the answer is:
weight
I believe this clue is a double definition.
'the importance' is the first definition.
(weight is a kind of importance)
'gravity' is the second definition.
(both can mean importance or significance)
'of' acts as a link.
(Other definitions for weight that I've seen before include "Something to lose" , "Pressure" , "Vertical force as a result of gravity" , "Gravitational pull" , "Burden" .)