Crossword Genius

A prod behind a big bird (5)

Ross

I believe the answer is:

goose

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

'a prod' is the definition.
(I know that goose is a kind of prod)

'behind a big bird' is the wordplay.
I cannot quite understand how this works, but
'big' could be 'os' (abbreviation for oversize) and 'os' is located in the answer.
The remaining letters 'goe' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for goose that I've seen before include "Gander" , "Silly person" , "Seasonal fare" , "Marches in an odd fashion" , "Large waterbird" .)

I've seen this clue in the Sydney Morning Herald.
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