Stone to kill, sent skywards (5)
I believe the answer is:
topaz
'stone' is the definition.
(I know that topaz is a type of transparent gem)
'to kill sent skywards' is the wordplay.
'kill' becomes 'zap' (I've seen this before).
'sent skywards' says the letters should be written backwards (in a down clue, letters go up).
'zap' reversed gives 'paz'.
'to'+'paz'='TOPAZ'
(Other definitions for topaz that I've seen before include "Silicate gemstone" , "Yellow or pale blue gem" , "Precious stone - dark yellow colour" , "valuable item" , "Yellowish-brown gemstone" .)