Ultimate death of wading bird: died after swallowing ecstasy (6,3)
I believe the answer is:
bitter end
'ultimate death' is the definition.
'wading bird died after swallowing ecstasy' is the wordplay.
'wading bird' becomes 'bittern' (bittern is a kind of wading bird**).
'died' becomes 'd'.
'after' says to put letters next to each other.
'swallowing' indicates putting letters inside.
'ecstasy' becomes 'e' (informal term for the drug).
'bittern'+'d'='bitternd'
'bitternd' enclosing 'e' is 'BITTER END'.
'of' is the link.
(Other definitions for bitter end that I've seen before include "Losers may persevere to it" , "Limit of one's resources" .)