Precious stone placed in top altar (4)
I believe the answer is:
opal
'precious stone' is the definition.
(I've seen this before)
'placed in top altar' is the wordplay.
'placed in' indicates the answer is hidden within the clue.
'OPAL' can be found hidden inside 'top altar'.
(Other definitions for opal that I've seen before include "Semi-transparent gem" , "Shimmering stone" , "Milky-white mineral" , "Lustrous stone" , "Type of gem" .)