Tough guys maybe apt to do it in (6)
I believe the answer is:
muscle
'in' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for muscle that I've seen before include "Contracting body part" , "Fibrous tissue with the ability to contract" , "Force; a body tissue" , "Physical power" , "contractor" .)