Mourn headless heron (5)
I believe the answer is:
egret
'heron' is the definition.
(egret is a kind of heron)
'mourn headless' is the wordplay.
'mourn' becomes 'regret' (I've seen this before).
'headless' means to remove the first letter.
'regret' with its initial letter removed is 'EGRET'.
(Other definitions for egret that I've seen before include "Heron-like wader" , "One wading" , "Greet (anag) -- wading bird" , "(Usually) white heron" , "Stork-like bird" .)