Small sum we might invest in American prospector (9)
I believe the answer is:
sourdough
'american prospector' is the definition.
(historical term for an American gold prospector)
'small sum we might invest' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'sum we might invest' becomes 'our dough' (dough belonging to us).
's'+'ourdough'='SOURDOUGH'
'in' is the link.
(Other definitions for sourdough that I've seen before include "Type of bread" , "Fermenting bread mixture - odour hugs (anag)" , "Leaven for making bread" .)