Valuable information about antique (6)
I believe the answer is:
golden
'valuable' is the definition.
(thesaurus)
'information about antique' is the wordplay.
'information' becomes 'gen' (gen is a kind of information).
'about' means one lot of letters goes inside another.
'antique' becomes 'old' ('old' can be similar in meaning to 'antique').
'gen' going around 'old' is 'GOLDEN'.
(Other definitions for golden that I've seen before include "Most favourable; bright" , "Gilded, pale yellow" , "Autumnal flower" , "- - wedding (50th)" , "happy" .)