Look chestnut horse over (6)
I believe the answer is:
gander
'look' is the definition.
(informal term for a look or glance)
'chestnut horse over' is the wordplay.
'chestnut' becomes 'red' (chestnut is a shade of red).
'horse' becomes 'nag' (term for an old horse).
'over' is a reversal indicator.
'red'+'nag'='rednag'
'rednag' back-to-front is 'GANDER'.
(Other definitions for gander that I've seen before include "town in Newfoundland" , "glance" , "Male goose; look (colloq.)" , "Ranged over the bird" , "'What's sauce for the goose is sauce for the . . . . . .'" .)