Part of an Uncle Sam costume (5)
I believe the answer is:
beard
'part of an uncle sam costume' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for beard that I've seen before include "Oppose with effrontery" , "Facial feature" , "SECOND PART OF 14" , "In good health; very" , "Goatee, for instance" .)