Fancy their sister, one with long hair (5,6)
I believe the answer is:
irish setter
'one with long hair' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Maybe there's an association between them I don't understand?
'fancy their sister' is the wordplay.
'fancy' indicates an anagram.
'their'+'sister'='theirsister'
'theirsister' with letters rearranged gives 'IRISH SETTER'.
Can you help me to learn more?
(Other definitions for irish setter that I've seen before include "Dog from the Emerald Isle" , "Kind of dog" , "best friend" , "Breed of dog" , "Dog with dark red coat" .)