Gravity of appeal for help by son investing capital (9)
I believe the answer is:
soberness
'gravity' is the definition.
('soberness' can be a synonym of 'gravity')
'appeal for help by son investing capital' is the wordplay.
'appeal for help' becomes 'sos' (I've seen this before).
'by' means one lot of letters go next to another.
'son' becomes 's'.
'investing' indicates putting letters inside.
'capital' becomes 'berne' (Berne is an example**).
'sos'+'s'='soss'
'soss' placed around 'berne' is 'SOBERNESS'.
'of' is the link.
(Another definition for soberness that I've seen is " Gravity".)