Doctor protecting source of tropical fish (8)
I believe the answer is:
sturgeon
'fish' is the definition.
(sturgeon is a kind of fish)
'doctor protecting source of tropical' is the wordplay.
'doctor' becomes 'surgeon' (surgeon is a kind of doctor**).
'protecting' means one lot of letters goes inside another.
'source of' suggests taking the first letters.
The initial letter of 'tropical' is 't'.
'surgeon' going around 't' is 'STURGEON'.
(Other definitions for sturgeon that I've seen before include "producer of expensive eggs" , "caviar suppliers" , "Fish from which caviar is obtained" , "So urgent to find caviar-bearing fish" , "Caviar supplier" .)