Whinge from signalmen caught by one dropping a line (6)

I believe the answer is:

salmon

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

'line' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for salmon that I've seen before include "Fish (often smoked)" , "Migratory fish" , "Oddly, Mal's no fish" , "high-jumper" , "Legendary fish of knowledge" .)

I've seen this clue in the Irish Times.

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.