Not a naval hero, but a fine swimmer (3,4)
I believe the answer is:
sea lion
'not a naval' is the definition.
I can't judge whether this definition defines the answer.
'hero but a fine swimmer' is the wordplay.
I cannot quite understand how this works, but
'hero' could be 'lion' and 'lion' is found in the answer.
'swimmer' could be 'sea' (sea fish is a kind of fish) and 'sea' is found in the answer.
No letters remain.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sea lion that I've seen before include "Large-eared marine mammal, sometimes performing" , "Marine animal, used as a circus performer" , "Large seal" , "Large marine mammal" , "one often immersed in salty water" .)