Black horses, first of three having escaped (5)
I believe the answer is:
sable
'black' is the definition.
(heraldic term for black)
'horses first of three having escaped' is the wordplay.
'horses' becomes 'stable' (I've seen this before).
'first of three' becomes 't' (1st letter of 'three').
'having escaped' suggests deleting specific letters.
'stable' with 't' removed is 'SABLE'.
(Other definitions for sable that I've seen before include "Marten's fur - dark black" , "Adventurous courage" , "Bales of expensive Russian fur" , "Antelope, fur" , "Fur - heraldic black" .)