Animal that's vanished as horse advanced (6)
I believe the answer is:
quagga
'animal' is the definition.
(extinct animal)
'as horse advanced' is the wordplay.
'as' becomes 'qua' ('qua' is a Latin term meaning 'in the capacity of').
'horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'advanced' becomes 'a' (abbreviation).
'qua'+'gg'+'a'='QUAGGA'
'that's vanished' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for quagga that I've seen before include "Extinct S. Afr. zebra" , "Extinct animal" , "old zebra" , "Extinct zebra relative" , "Extinct wild ass" .)