Reckon year with investment of gold is a good one (6)
I believe the answer is:
beauty
'a good one' is the definition.
(a fine example of something may be called a beauty)
'reckon year with investment of gold' is the wordplay.
'reckon' becomes 'bet' (bet can mean to assume or reckon something is true).
'year' becomes 'y'.
'with investment of' indicates putting letters inside (letters are inserted or invested).
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'bet'+'y'='bety'
'bety' enclosing 'au' is 'BEAUTY'.
'is' acts as a link.
(Other definitions for beauty that I've seen before include "very attractive lady" , "Proverbially, it's in the eye of the beholder" , "it delights the senses" , "Truth, for Keats; She walks in it, Byron" , "- Salon; - spot" .)