Goliath, for instance, was no stag! (6)

I believe the answer is:

beetle

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

'stag' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for beetle that I've seen before include "children's game" , "Insect with hardened forewings - scurry" , "Lee bet he could be an insect" , "E.g. ladybird" , "One as ladybird, weevil" .)

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.