Shop always displays precious stone (4)
I believe the answer is:
opal
'precious stone' is the definition.
(I've seen this before)
'shop always displays' is the wordplay.
'displays' says the answer is hidden in the clue (part of the clue displays the answer).
'OPAL' can be found hidden inside 'shop always'.
(Other definitions for opal that I've seen before include "Milky stuff that's hard" , "Variegated gemstone" , "Form of hydrated silica, often used as a gemstone" , "item in Hatton Garden?" , "Semiprecious stone of white and bluish colour" .)