It's a mistake to greet two geese thus! (6)
I believe the answer is:
booboo
'it's a mistake' is the definition.
(informal term for a mistake)
'two geese thus' is the wordplay.
'two' says to double the letters.
'geese thus' becomes 'boo' (I am not sure about this - if you are sure you should believe this answer much more).
'boo' duplicated is 'BOOBOO'.
'to greet' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for booboo that I've seen before include "Error" , "Blunder" , "mistake" , "Having got animated, bear" .)