Small pistol (4)
(Other definitions for colt that I've seen before include "Youngster" , "Mount" , "runner" , "Rod" , "Early American revolver" .)
'small pistol' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for derringer that I've seen before include "Small pistol used by gamblers in western films" , "Pocket pistol" , "Weapon" , "Short-barrelled American pistol" .)