Angry lie about racehorse (8)
I believe the answer is:
yearling
'racehorse' is the definition.
(I know that yearling is a type of racehorse)
'angry lie about' is the wordplay.
'about' indicates anagramming the letters.
'angry'+'lie'='angrylie'
'angrylie' with letters rearranged gives 'YEARLING'.
(Other definitions for yearling that I've seen before include "Youngster" , "Young racehorse" , "Twelve-month old animal" , "Sort of racehorse" , "Animal 12 months old" .)