Zebra or horse in aqua perhaps (6)
I believe the answer is:
quagga
'zebra' is the definition.
(I have seen 'old zebra' mean 'quagga' so perhaps 'zebra' could also mean 'quagga')
'horse in aqua perhaps' is the wordplay.
'horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'in' means one lot of letters goes inside another.
'perhaps' indicates anagramming the letters.
'aqua' with letters rearranged gives 'quaa'.
'gg' put within 'quaa' is 'QUAGGA'.
'or' is the link.
(Other definitions for quagga that I've seen before include "Extinct animal" , "Extinct wild ass" , "Extinct S. Afr. zebra" , "Extinct zebra relative" , "old zebra" .)