A shipmaster, when not at sea, ordered seafood (6)

I believe the answer is:

shrimp

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

'seafood' is the definition.
(shrimp is a kind of seafood)

I don't understand how the rest of the clue works.

(Other definitions for shrimp that I've seen before include "Runt" , "very small one" , "Raising of a siege" , "School member" , "Diminutive person" .)

I've seen this clue in The Guardian.

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.