Old knight beheaded model (8)
I believe the answer is:
exemplar
'model' is the definition.
(synonyms)
'old knight beheaded' is the wordplay.
I cannot really see how this works, but
'old' could be 'ex' (prefix meaning former or onetime) and 'ex' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for exemplar that I've seen before include "Epitome" , "Typical specimen" , "Ideal model" , "Something serving as an appropriate model" , "A good model or type" .)