Stones thrown by mere lads (8)
I believe the answer is:
emeralds
'stones' is the definition.
(I've seen this before)
'thrown by mere lads' is the wordplay.
'thrown' indicates anagramming the letters.
'by' is a charade indicator (letters next to each other).
'mere' after 'lads' is 'ladsmere'.
'ladsmere' anagrammed gives 'EMERALDS'.
(Other definitions for emeralds that I've seen before include "Mere lads (anag.)" , "Colours" , "Green gems" , "Green stones from strangely red males" , "Precious stones for odd red males" .)