Old style piece found in treasure trove (5)
I believe the answer is:
retro
'old style' is the definition.
'piece found in treasure trove' is the wordplay.
'piece found in' says the answer is hidden in the clue.
'RETRO' can be found hidden inside 'treasure trove'.
(Other definitions for retro that I've seen before include "Associated with the past" , "Looking back" , "Prefix for old-style" , "in the style of yesteryear" , "reminding us of the past" .)