Rich source, fish served with last drops of sardine oil
I believe the answer is:
el dorado
'rich source' is the definition.
(mythical city of gold)
'fish served with last drops of sardine oil' is the wordplay.
'fish' becomes 'dorado' (another name for the mahi-mahi fish**).
'served with' is a charade indicator (letters next to each other).
'last drops of' says to take the final letters.
The final letters of 'sardine oil' are 'el'.
'dorado' after 'el' is 'EL DORADO'.
(Other definitions for el dorado that I've seen before include "Fabled rich land" , "Mythical land of gold" , "Imaginary place of great wealth in South America" , "One who can't give something up" , "A doodler in fabled land of gold" .)