Very cold, heron-like bird emitting note (6)
I believe the answer is:
bitter
'very cold' is the definition.
(bitter informally can mean very cold)
'heron-like bird emitting note' is the wordplay.
'heron-like bird' becomes 'bittern' (bird of the heron family).
'emitting' suggests deleting specific letters.
'note' becomes 'n' (abbreviation).
'bittern' with 'n' removed is 'BITTER'.
(Other definitions for bitter that I've seen before include "Sour taste or resentful" , "Tart like some English beer" , "Hard to accept" , "Acrimonious; beer" , "English ale, sounds sour" .)