Possibly it arrests one guilty of treason (9)
I believe the answer is:
traitress
'guilty of treason' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'possibly it arrests one' is the wordplay.
'possibly' indicates an anagram.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'one' becomes 'i' (Roman numeral).
't'+'arrests'+'i'='tarrestsi'
'tarrestsi' with letters rearranged gives 'TRAITRESS'.
Can you help me to learn more?
(Other definitions for traitress that I've seen before include "False woman" , "Betrayer of her country" .)