Sociable guy at first argues about investing old capital (10)
I believe the answer is:
gregarious
'sociable' is the definition.
(I've seen this before)
'guy at first argues about investing old capital' is the wordplay.
'guy at first' becomes 'g' (I am not sure about this - if you are sure you should believe this answer much more).
'about' indicates anagramming the letters.
'investing' means one lot of letters goes inside another (inserted letters are taken in or invested).
'old capital' becomes 'rio' (I've seen this before).
'argues' with letters rearranged gives 'regaus'.
'regaus' enclosing 'rio' is 'regarious'.
'g'+'regarious'='GREGARIOUS'
Can you help me to learn more?
(Other definitions for gregarious that I've seen before include "Outgoing" , "Sociable" , "Enjoying the company of others" , "Living in flocks" , "tending to flock" .)