Small robin primarily found in brownish-purple tree (6)
I believe the answer is:
spruce
'tree' is the definition.
(spruce is a kind of tree)
'small robin primarily found in brownish-purple' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'primarily' indicates taking the first letters.
'found in' means one lot of letters goes inside another.
'brownish-purple' becomes 'puce' (I've seen this before).
The initial letter of 'robin' is 'r'.
'r' going within 'puce' is 'pruce'.
's'+'pruce'='SPRUCE'
(Other definitions for spruce that I've seen before include "Smart-looking sort of fir tree" , "wood" , "ornamental tree" , "Neat and dapper tree" , "Well-groomed - tree" .)