Grade 1C is a standard of excellence (7)
I believe the answer is:
classic
'standard of excellence' is the definition.
'classic' can be an answer for 'standard' (thesaurus). I'm not sure about the 'of excellence' bit.
'grade 1c' is the wordplay.
'grade' becomes 'class' (synonyms).
'ic' becomes 'ic' (I can't justify this - if you can you should believe this answer much more).
'class'+'ic'='CLASSIC'
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for classic that I've seen before include "Artistic work of lasting excellence" , "Judged over a long period to be of high quality" , "Traditional" , "Work of enduring excellance" , "Oaks perhaps" .)