Preakness horse's age (5)
I believe the answer is:
three
'preakness horse's age' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for three that I've seen before include "Number of Furies or Norns" , "One, one, one" , "Number constituting a crowd?" , "... number of his "Steps to Heaven"" , "Subject of 1 9" .)