A mine in California yielding wealth (7)
I believe the answer is:
capital
'wealth' is the definition.
(I've seen this before)
'a mine in california' is the wordplay.
'mine' becomes 'pit' (down the pit).
'in' indicates putting letters inside.
'california' becomes 'cal'.
'a'+'pit'='apit'
'apit' put inside 'cal' is 'CAPITAL'.
'yielding' acts as a link.
(Other definitions for capital that I've seen before include "Havana, say" , "A great city - that's fine" , "Principal city" , "Big money?" , "Money available to produce interest" .)