Former knight abandoning leader's standard (8)
I believe the answer is:
exemplar
'standard' is the definition.
(thesaurus)
'former knight abandoning leader's' is the wordplay.
'former' becomes 'ex'.
'knight' becomes 'templar' (Templar is an example).
'abandoning leader' means to remove the first letter.
'templar' with its first letter removed is 'emplar'.
'ex'+'emplar'='EXEMPLAR'
(Other definitions for exemplar that I've seen before include "Model to be followed" , "A model or instance" , "Something serving as an appropriate model" , "A good model or type" , "Epitome" .)