A mine in California is excellent! (7)
I believe the answer is:
capital
'excellent' is the definition.
(thesaurus)
'a mine in california' is the wordplay.
'mine' becomes 'pit' (down the pit).
'in' means one lot of letters goes inside another.
'california' becomes 'cal'.
'a'+'pit'='apit'
'apit' going inside 'cal' is 'CAPITAL'.
'is' is the link.
(Other definitions for capital that I've seen before include "Topping" , "Most important city" , "A big letter, or a big city" , "A country's main city" , "savings" .)