Good past making one happy (6)
I believe the answer is:
golden
'one happy' is the definition.
(I've seen this before)
'good past' is the wordplay.
'good' becomes 'g' (abbreviation).
'past' becomes 'olden' (similar in meaning).
'g'+'olden'='GOLDEN'
'making' is the link.
(Other definitions for golden that I've seen before include "Excellent" , "Most favourable; bright" , "Valuable" , "happy" , "Colour of the fleece sought by Jason" .)