Highly desirable to get on down etc (7)
I believe the answer is:
plumage
'down etc' is the definition.
(down can mean a bird's feathers)
'highly desirable to get on' is the wordplay.
'highly desirable' becomes 'plum' (plum can mean excellent).
'to get on' becomes 'age' (synonyms).
'plum'+'age'='PLUMAGE'
(Other definitions for plumage that I've seen before include "Birds' clothing" , "Bird's covering of feathers" , "A bird's feathers, may be fancy" , "Cover for diver" , "One's feathers" .)