Fool (around) (5)
I believe the answer is:
horse
'fool around' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for horse that I've seen before include "Equine quadruped" , "Vaulting --; -- sense" , "Cavalry - gymnastic apparatus" , "Animal featuring in the sport of kings" , "Equid" .)