Sign of rust developing around gold (6)
I believe the answer is:
taurus
'sign' is the definition.
(Taurus is an example)
'rust developing around gold' is the wordplay.
'developing' is an anagram indicator.
'around' means one lot of letters goes inside another.
'gold' becomes 'au' (Au is the chemical symbol for gold).
'rust' anagrammed gives 'trus'.
'trus' going around 'au' is 'TAURUS'.
'of' acts as a link.
(Other definitions for taurus that I've seen before include "Bull constellation" , "April/May sign" , "Zodiac sign - constellation" , "Sign of the Bull from 12 across" , "A group of stars" .)