Couple of scoundrels, second of which knocking out a tool (7)
I believe the answer is:
ratchet
'tool' is the definition.
(type of tool or device)
I cannot understand how the rest of the clue works.
(Other definitions for ratchet that I've seen before include "Device preventing backward motion" , "Device promoting One Direction" , "One-way-motion toothed wheel" , "Device allowing motion in one direction only" , "One-way toothed wheel" .)