Second word of the golden rule (4)
I believe the answer is:
unto
'second word of the golden rule' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for unto that I've seen before include "To old" , "In the direction of (archaic)" , "Heading for" , "'''For ... us a child is born'' (Handel's Messiah) (4)'" , "To (obs.)" .)