The horse raced round the circuit (4)
I believe the answer is:
roan
'the horse' is the definition.
(roan is a kind of horse)
'raced round the circuit' is the wordplay.
'raced' becomes 'ran' ('run' can be a synonym of 'race').
'round' means one lot of letters goes inside another.
'the circuit' becomes 'o'.
'ran' going around 'o' is 'ROAN'.
(Other definitions for roan that I've seen before include "Of horse, with coat sprinkled with white or grey hairs" , "nag" , "Nora (anag) - horse" , "Horse with coat speckled with white or grey" , "Horse with main colour interspersed with hairs of another" .)