Barking swimmer? (7)

I believe the answer is:

dogfish

I'm a little stuck... Click here to teach me more about this clue!

'swimmer?' is the definition.
(dogfish is a kind of aquatic vertebrate)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for dogfish that I've seen before include "Small, edible shark" , "Nursehound" , "'Small shark, sounds canine (7)'" , "Restaurants call it rock salmon" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.