Happy anniversary! (6)
I believe the answer is:
golden
This clue is a double definition.
'happy' is the first definition.
(similar in meaning)
'anniversary' is the second definition.
(as in a golden anniversary)
(Other definitions for golden that I've seen before include "Halcyon - yellow" , "With sixteen across Apple variety" , "Excellent" , "Gilded, pale yellow" , "happy" .)