Ancient splendour of American standard (3,5)
I believe the answer is:
old glory
'american standard' is the definition.
(I've seen this in another clue)
'ancient splendour' is the wordplay.
'ancient' becomes 'old' (similar in meaning).
'splendour' becomes 'glory' (glory can mean splendour or grandeur).
'old'+'glory'='OLD GLORY'
'of' acts as a link.
(Other definitions for old glory that I've seen before include "Nickname for US flag" , "National flag of the United States" , "seen flying at pole" , "The Stars and Stripes" , "Nickname for the flag of the United States" .)