A through train ultimately for the birds? (5)

I believe the answer is:

avian

'for the birds?' is the definition.
(avian means relating to birds)

'a through train ultimately' is the wordplay.
'through' becomes 'via' (I've seen this before).
'ultimately' suggests the final letters.
The final letter of 'train' is 'n'.
'a'+'via'+'n'='AVIAN'

(Other definitions for avian that I've seen before include "Like a booby" , "To do with birds - like a dangerous flu" , "Resembling a bird" , "Regarding birds" , "Relating to birds" .)

I've seen this clue in The Times.

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