Shop alarm protecting gem (4)
I believe the answer is:
opal
'gem' is the definition.
(I know that opal is a type of opaque gem)
'shop alarm protecting' is the wordplay.
'protecting' indicates the answer is hidden within the clue.
'OPAL' is hidden in the letters of 'shop alarm'.
(Other definitions for opal that I've seen before include "Semi-precious stone of milky or bluish colour" , "Many-coloured gem" , "Birthstone for October" , "Glassy gemstone" , "Delicately coloured gem" .)