Excellent leader trained dog (8)
I believe the answer is:
airedale
'dog' is the definition.
(Airedale is an example)
'excellent leader trained' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'leader' anagrammed gives 'redale'.
'ai'+'redale'='AIREDALE'
(Other definitions for airedale that I've seen before include "Thematic type [DOG]" , "part of Yorkshire" , "Terrier - Yorkshire area" , "Dog taking" , "Yorkshire valley" .)