Mere lads, rolling stones (8)
I believe the answer is:
emeralds
'stones' is the definition.
(I've seen this before)
'mere lads rolling' is the wordplay.
'rolling' indicates an anagram.
'mere'+'lads'='merelads'
'merelads' with letters rearranged gives 'EMERALDS'.
(Other definitions for emeralds that I've seen before include "Green gems" , "They're preciously green on the Isle" , "Colours" , "Precious stones for odd red males" , "Mere lads (anag.)" .)