Part of treasure trove is consciously old-fashioned (5)
I believe the answer is:
retro
'consciously old-fashioned' is the definition.
(I've seen this before)
'part of treasure trove is' is the wordplay.
'part of' says the answer is hidden in the clue.
'RETRO' can be found hidden inside 'treasure trove is'.
(Other definitions for retro that I've seen before include "Imitating an older style" , "Of a past time" , "Slangy adjective for harking back to the past" , "reminding us of the past" , "Associated with the past" .)