Stone heads of organisation promoting antiquated laws (4)
I believe the answer is:
opal
'stone' is the definition.
(I've seen this before)
'heads of organisation promoting antiquated laws' is the wordplay.
'heads of' suggests taking the first letters.
The initial letters of 'organisation promoting antiquated laws' is 'OPAL'.
(Other definitions for opal that I've seen before include "Semiprecious stone of white and bluish colour" , "Translucent gem" , "National gemstone of Australia" , "Milky-white mineral" , "Gem-stone with a milky quality" .)