Gravity of appeal for help by son investing capital (9)
I believe the answer is:
Here is my best explanation:
'gravity' is the definition.
('soberness' can be a synonym of 'gravity')
'appeal for help by son investing capital' is the subsidiary indication.
'appeal for help' becomes 'sos' (I've seen this before).
'by' means one lot of letters go next to another.
'son' becomes 's'.
'investing' is an insertion indicator.
'capital' becomes 'berne' (Berne is an example**).
'soss' placed around 'berne' is 'soberness'.
'of' is the link.
(Another definition for soberness that I've seen is "Gravity".)