Bird: it nested in empty estuary for ages (8)
I believe the answer is:
eternity
'ages' is the definition.
(thesaurus)
'bird it nested in' is the wordplay.
'bird' becomes 'tern' (tern is a kind of bird).
'nested' becomes 'ey' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'tern'+'it'='ternit'
'ternit' going inside 'ey' is 'ETERNITY'.
'empty estuary for' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for eternity that I've seen before include "timelessness" , "Entirety (anag.)" , "Endless duration" , "I try teen (anag.)" , "Time without end" .)