Red jewel (4)
I believe the answer is:
ruby
'red jewel' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ruby that I've seen before include "Gemstone - shade of red" , "Gemstone comprising mainly aluminium oxide" , "Fortieth anniversary stone" , "Girl's name - colour" , "Stony girl" .)