Naval doctor describing head of terrifying fish (8)
I believe the answer is:
sturgeon
'fish' is the definition.
(I know that sturgeon is a fish)
'naval doctor describing head of terrifying' is the wordplay.
'naval doctor' becomes 'surgeon' (surgeon can mean a doctor within the navy).
'describing' indicates putting letters inside (describe can mean to form the outline of).
'head of' suggests taking the first letters.
The initial letter of 'terrifying' is 't'.
'surgeon' enclosing 't' is 'STURGEON'.
(Other definitions for sturgeon that I've seen before include "So urgent to find caviare-bearing fish" , "producer of expensive eggs" , "provider of delicacy?" , "Caviar-producing fish" , "Main resident" .)