Add a PS to letter about explosive matter (9)
I believe the answer is:
saltpetre
'add a ps' is the definition.
I can't tell whether this defines the answer.
'letter about explosive matter' is the wordplay.
I cannot quite see how this works, but
'letter' could be 'pe' (pe is a kind of letter) and 'pe' is located in the answer.
'about' could be 're' (regarding) and 're' is located in the answer.
'matter' could be 'salt' (salt is a kind of matter) and 'salt' is found in the answer.
A single letter 't' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'to' is the 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" , "Nitre" , "Gunpowder ingredient" , "chemical" , "Component of gunpowder" .)