Grand antique could be precious (6)
I believe the answer is:
golden
'precious' is the definition.
(thesaurus)
'grand antique could' is the wordplay.
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'antique' becomes 'old' ('old' can be similar in meaning to 'antique').
'could' becomes 'en' (I can't justify this - if you can you should give a lot more credence to this answer).
'g'+'old'+'en'='GOLDEN'
'be' is the link.
Can you help me to learn more?
(Other definitions for golden that I've seen before include "- - handshake" , "Autumnal flower" , "Very favourable (opportunity); aureate" , "Halcyon - yellow" , "With sixteen across Apple variety" .)