A letter P's incorrectly signifying potassium compound (9)
I believe the answer is:
saltpetre
'compound' is the definition.
(saltpetre is a kind of compound)
'a letter p's incorrectly' is the wordplay.
'incorrectly' is an anagram indicator (I've seen this in other clues).
'a'+'letter'+'ps'='aletterps'
'aletterps' is an anagram of 'SALTPETRE'.
'signifying potassium' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for saltpetre that I've seen before include "Nitre" , "It's used as a fertilizer and explosive" , "Potassium nitrate" , "Compound" , "Gunpowder ingredient" .)