Explosive content of letter and a PS that's outrageous (9)
I believe the answer is:
saltpetre
'explosive content' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'letter and a ps that's outrageous' is the wordplay.
I cannot quite understand how this works, but
'letter' could be 'pe' (pe is a kind of letter of the alphabet) and 'pe' is found within the answer.
'a' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'of' acts as a link.
Can you help me to learn more?
(Other definitions for saltpetre that I've seen before include "It's used as a fertilizer and explosive" , "Constituent of gunpowder" , "Component of gunpowder" , "Nitre" , "Explosive ingredient" .)