It's eaten with goose and straw (5)
I believe the answer is:
berry
'goose and straw' is the definition.
I can't judge whether this defines the answer.
'it's eaten with' is the wordplay.
'it's' becomes 'err' (I can't justify this - if you can you should give a lot more credence to this answer).
'eaten' means one lot of letters goes inside another.
'with' becomes 'by' (both can mean close together).
'err' put inside 'by' is 'BERRY'.
Can you help me to learn more?
(Other definitions for berry that I've seen before include "Small succulent fruit" , "Word linking black or blue" , "'One might be straw, rasp or blue, say (5)'" , "Small edible fruit" , "Pulpy edible fruit" .)