Some regretful bird (5)
I believe the answer is:
egret
'bird' is the definition.
(egret is a kind of bird)
'some regretful' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'EGRET' is hidden within 'regretful'.
(Other definitions for egret that I've seen before include "Heron-like wader" , "one with plume" , "One of a pair of waders" , "(Usually) white heron" , "Wading bird like a heron" .)