Aquatic bird -- it nested within banks of estuary for donkey's years (8)
I believe the answer is:
eternity
'donkey's years' is the definition.
(donkey's years means a very long time)
'aquatic bird it nested within banks of estuary' is the wordplay.
'aquatic bird' becomes 'tern' (tern is a kind of aquatic bird).
'nested within' means one lot of letters goes inside another.
'banks of' says to hollow out the word (remove centre letters) (the banks are the sides of the word).
'estuary' with its centre removed is 'ey'.
'tern'+'it'='ternit'
'ternit' placed within 'ey' is 'ETERNITY'.
'for' acts as a link.
(Other definitions for eternity that I've seen before include "Evermore" , "An extremely long time" , "Endless time" , "Sort of ring" , "Type of ring" .)