Feather in one's cap (5)
I believe the answer is:
plume
'feather in one's cap' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for plume that I've seen before include "Feather; column of smoke" , "Large showy feather" , "Smoke when rising" , "A fancy feather" , "Conspicuous feather of a bird" .)