Reddish-purple feather loses a point (4)
I believe the answer is:
plum
'reddish-purple' is the definition.
(I've seen this in another clue)
'feather loses a point' is the wordplay.
'feather' becomes 'plume' (synonyms).
'loses' indicates named letters should be taken away.
'a point' becomes 'e' (compass point).
'plume' with 'e' removed is 'PLUM'.
(Other definitions for plum that I've seen before include "Dark-red fruit" , "First-class - fruit" , "Select" , "Purple fruit" , "Fruit tree - of especial value?" .)