Head off, dropping strange red stone (6)
I believe the answer is:
garnet
'red stone' is the definition.
(I've seen this in another clue)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for garnet that I've seen before include "Semi-precious stone of deep transparent red" , "Semi-precious stone of argent type" , "Precious stone, usually red" , "Glassy red, yellow or green gemstone" , "Hard glassy mineral used as a gemstone" .)