Bird in sorrow hiding head (5)
I believe the answer is:
egret
'bird' is the definition.
(egret is a kind of bird)
'sorrow hiding head' is the wordplay.
'sorrow' becomes 'regret' (regret is a kind of sorrow).
'hiding head' means to remove the first letter.
'regret' with its initial letter taken away is 'EGRET'.
'in' is the link.
(Other definitions for egret that I've seen before include "Seabird" , "White wading-bird" , "One of a pair of waders" , "flier" , "(Usually) white heron" .)