Ageless waterbird feeds another, crestfallen? (7)

I believe the answer is:

eternal

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'ageless' is the definition.
(synonyms)

'another crestfallen?' is the wordplay.
'another' becomes 'e' (I've seen this in other clues).
'crestfallen?' becomes 'ternal' (I can't explain this - if you can you should believe this answer much more).
'e'+'ternal'='ETERNAL'

'waterbird feeds' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for eternal that I've seen before include "Such a city is Rome" , "non-stop" , "Lasting forever, tediously persistent" , "Infinite - endless" , "Imperishable" .)

I've seen this clue in the Sydney Morning Herald.

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