A matter of some gravity (6)
I believe the answer is:
weight
'gravity' is the definition.
(both can mean importance or significance)
'a matter of some' is the wordplay.
'a matter' becomes 'W' (W is an example).
'of some' becomes 'eight' (I can't justify this - if you can you should give a lot more credence to this answer).
'w'+'eight'='WEIGHT'
Can you help me to learn more?
(Other definitions for weight that I've seen before include "Import" , "Pressure" , "Relative heaviness" , "gives a sense of oppression" , "Gravitational pull" .)