Whitish gem (4)
I believe the answer is:
opal
'whitish gem' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for opal that I've seen before include "what may be set in 29 [DIAL]" , "Gem stone" , "Delicately coloured gem" , "Opaque mineral used a gemstone" , "Milky white gemstone" .)