Stone delivered from them by cockney messenger (7)
I believe the answer is:
emerald
'stone' is the definition.
(type of gemstone)
'them by cockney messenger' is the wordplay.
'them' becomes 'em' (abbreviation).
'by' is a charade indicator (letters next to each other).
'cockney messenger' becomes ''erald' ('herald' - herald is a kind of messenger - said with the 'H' dropped - a cockney accent.).
'em'+'erald'='EMERALD'
'delivered from' acts as a link.
(Other definitions for emerald that I've seen before include "Bright green colour" , "...re medal? (Anag)" , "Led mare (anag.)" , "Highly-valued variety of beryl" , "Green gemstone" .)