No great swimmer, especially when canned! (7)

I believe the answer is:

sardine

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

'no great swimmer' is the definition.
'sardine' can be an answer for 'swimmer' (sardine is a kind of fish). I am unsure of the 'no great' bit.

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

Can you help me to learn more?

(Other definitions for sardine that I've seen before include "Fish in tin?" , "Fish (often tinned)" , "Small pilchard" , "FRENCH FISH" , "(Cramped?) fish" .)

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.