Butcher's chestnut horse turned up (6)
I believe the answer is:
gander
'butcher's' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'chestnut horse turned up' is the wordplay.
'chestnut' becomes 'red' (chestnut is a shade of red).
'horse' becomes 'nag' (term for an old horse).
'turned up' says the letters should be written in reverse.
'red'+'nag'='rednag'
'rednag' backwards is 'GANDER'.
Can you help me to learn more?
(Other definitions for gander that I've seen before include "'What's sauce for the goose is sauce for the . . . . . .'" , "Glimpse" , "Male goose; look (colloq.)" , "male member of the Anatidae family" , "town in Newfoundland" .)