Gander's mate (5)
I believe the answer is:
goose
'gander's mate' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for goose that I've seen before include "Waterbird; fool" , "Bird; foolish person" , "Bird - tailor's iron" , "Gander's mate" , "Silly person" .)