Butcher's chestnut horse should be returned (6)
I believe the answer is:
gander
'butcher's' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'chestnut horse should be returned' is the wordplay.
'chestnut' becomes 'red' (chestnut is a shade of red).
'horse' becomes 'nag' (term for an old horse).
'should be returned' says the letters should be written backwards.
'red'+'nag'='rednag'
'rednag' reversed gives 'GANDER'.
(Other definitions for gander that I've seen before include "Butchers - goose" , "Ranged over the bird" , "Male goose; look (colloq.)" , "town in Newfoundland" , "Glimpse" .)