Sphere of gold plus a bit of brass (3)
I believe the answer is:
orb
'sphere' is the definition.
(I know that orb is a type of sphere)
'gold plus a bit of brass' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'plus' says to put letters next to each other.
'a bit of' indicates taking the first letters.
The first letter of 'brass' is 'b'.
'or'+'b'='ORB'
'of' is the link.
(Other definitions for orb that I've seen before include "The World" , "Heavenly body" , "ball with cross" , "Poetically eye" , "Sphere for a sovereign" .)