Swallow's plumage (4)
I believe the answer is:
down
I believe this is a double definition.
'swallow's' is the first definition.
(I've seen this before)
'plumage' is the second definition.
(I know that down is a type of plumage)
(Other definitions for down that I've seen before include "Sad; feathers" , "like the other set of clues" , "In a low mood; hill" , "See 18" , "Depressed - drink" .)