Dark horse (3)
I believe the answer is:
bay
'dark horse' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for bay that I've seen before include "Indentation of a shoreline" , "Tree; horse; bark loudly" , "laurel maybe" , "Curved inlet; tree" , "Botany, Galway or Tampa" .)