Broken nail lost horse (8)
I believe the answer is:
stallion
'horse' is the definition.
(I know that stallion is a type of horse)
'broken nail lost' is the wordplay.
'broken' indicates an anagram.
'nail'+'lost'='naillost'
'naillost' anagrammed gives 'STALLION'.
(Other definitions for stallion that I've seen before include "Stud" , "Uncastrated adult male horse" , "Male horse, often used for breeding" , "Mount" , "Male horse kept for breeding" .)