White marsh bird (5)
I believe the answer is:
egret
'white marsh bird' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for egret that I've seen before include "White, plumed waterbird" , "Type of forty eight Ac" , "(Usually) white heron" , "young bird" , "White wading-bird" .)