Explosive ingredient key in spirit, head of course blowing off (9)
I believe the answer is:
saltpetre
'explosive ingredient' is the definition.
(I've seen this before)
'key in spirit head of course blowing off' is the wordplay.
I cannot quite see how this works, but
'key' could be 'alt' (the Alt keyboard button) and 'alt' is found in the answer.
'course' could be 's' and 's' is found within the answer.
The remaining letters 'petre' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for saltpetre that I've seen before include "Constituent of gunpowder" , "Compound" , "Potassium nitrate" , "It's used as a fertilizer and explosive" , "Explosive ingredient" .)