Nostalgic style in treasure trove (5)
I believe the answer is:
retro
'nostalgic style' is the definition.
(I've seen this in another clue)
'in treasure trove' is the wordplay.
'in' says the answer is hidden in the clue.
'RETRO' is hidden in the letters of 'treasure trove'.
(Other definitions for retro that I've seen before include "Imitative of past styles" , "Style derived from the past" , "Looking backwards" , "In a backward-looking style" , "Imitating a style from the recent past" .)