Diamonds in gold for one timeless decoration (5)
I believe the answer is:
medal
'timeless decoration' is the definition.
'medal' can be an answer for 'decoration' (synonyms). I'm not sure about the 'timeless' bit.
'diamonds in gold for one' is the wordplay.
'diamonds' becomes 'd' (abbreviation for the card suit diamonds).
'in' indicates putting letters inside.
'gold' becomes 'al' (I can't justify this - if you can you should believe this answer much more).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'one' becomes 'me' (a setter might use 'one' to humourously mean 'me').
'al' after 'me' is 'meal'.
'd' put inside 'meal' is 'MEDAL'.
Can you help me to learn more?
(Other definitions for medal that I've seen before include "Lamed for a gong" , "Religious or commenorative metal piece" , "Commemorative disc" , "sort of play" , "Small metal disc as reward or prize" .)