Happened upon old carving (5)
I believe the answer is:
cameo
'carving' is the definition.
(cameo is a kind of carving)
'happened upon old' is the wordplay.
'happened' becomes 'came' (coming is a kind of happening).
'upon' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'came'+'o'='CAMEO'
(Other definitions for cameo that I've seen before include "Carved head on gem - short dramatic role for star" , "Short performance" , "Part in movie" , "Medallion with a profile carved in relief" , "Star's tiny role" .)