Highest standard, good and ancient (4)
I believe the answer is:
gold
'highest standard' is the definition.
(gold standard)
'good and ancient' is the wordplay.
'good' becomes 'g' (abbreviation).
'and' is a charade indicator (letters next to each other).
'ancient' becomes 'old' (similar in meaning).
'g'+'old'='GOLD'
(Other definitions for gold that I've seen before include "Woman after a man's money" , "Element (Au)" , "Mine produces this" , "first gets this?" , "Valuable metal" .)