Gems editor’s penning lamer after revision (8)
I believe the answer is:
emeralds
'gems' is the definition.
(emerald is a kind of gem)
'editor's penning lamer after revision' is the wordplay.
'editor's' becomes 'eds'.
'penning' means one lot of letters goes inside another.
'after revision' indicates anagramming the letters (I've seen 'after revisions' mean this (I've seen this in other clues)).
'lamer' with letters rearranged gives 'meral'.
'eds' going around 'meral' is 'EMERALDS'.
(Other definitions for emeralds that I've seen before include "Green stones from strangely red males" , "They're preciously green on the Isle" , "Mere lads (anag.)" , "Colours" , "Green gems" .)