Pawn nobleman's gems (6)
I believe the answer is:
pearls
'gems' is the definition.
(pearl is a kind of gem)
'pawn nobleman's' is the wordplay.
'pawn' becomes 'p'.
'nobleman's' becomes 'earls' (earl is a kind of nobleman).
'p'+'earls'='PEARLS'
(Other definitions for pearls that I've seen before include "Lustrous round structures found in sea-creatures" , "Jewels from bi-valves" , "Lustrous structure in an oyster" , "Gemstones produced by molluscs" , "Nacreous gems" .)