Revolting leftist holds possibly criminal pistol (9)
I believe the answer is:
derringer
'pistol' is the definition.
(I know that Derringer is a type of pistol)
'revolting leftist holds possibly criminal' is the wordplay.
'revolting' is a reversal indicator.
'leftist' becomes 'red' (red can informally mean communist).
'holds' means one lot of letters go next to another.
'possibly criminal' becomes 'ringer' (I am not sure about this - if you are sure you should believe this answer much more).
'red' reversed gives 'der'.
'der'+'ringer'='DERRINGER'
Can you help me to learn more?
(Other definitions for derringer that I've seen before include "Weapon" , "Short-barrelled American pistol" , "Small pistol used by gamblers in western films" , "Pocket pistol" .)