Bird of umpteen colours, originally seen in brown (6)
I believe the answer is:
toucan
'bird' is the definition.
(I know that toucan is a type of piciform bird)
'of umpteen colours originally seen in brown' is the wordplay.
'originally' says to take the initial letters.
'seen in' means one lot of letters goes inside another.
'brown' becomes 'tan' (tan is a kind of brown).
The first letters of 'of umpteen colours' is 'ouc'.
'ouc' placed into 'tan' is 'TOUCAN'.
(Other definitions for toucan that I've seen before include "One with huge bill" , "Large-beaked bird" , "Brightly-coloured tropical bird with large beak" , "A pair could be able for this bird, by the sound of it" , "Colourful bird, big bill" .)