A gem some think too pale (4)
I believe the answer is:
opal
'a gem' is the definition.
(I know that opal is a type of opaque gem)
'some think too pale' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'OPAL' is hidden in the letters of 'think too pale'.
(Other definitions for opal that I've seen before include "Iridescent quartz-like stone" , "Birthstone for October" , "precious jewel" , "Semi-transparent gemstone" , "Many-coloured gem" .)