Naval doctor catches first of tuna, large fish (8)
I believe the answer is:
sturgeon
'large fish' is the definition.
(I've seen this before)
'naval doctor catches first of tuna' is the wordplay.
'naval doctor' becomes 'surgeon' (I've seen this before).
'catches' means one lot of letters goes inside another.
'first of' indicates taking the first letters.
The first letter of 'tuna' is 't'.
'surgeon' enclosing 't' is 'STURGEON'.
(Other definitions for sturgeon that I've seen before include "Caviar supplier" , "Caviare-producing fish" , "swimmer" , "producer of caviar" , "So urgent to find caviare-bearing fish" .)