Local thug would be legless without it (6)
I believe the answer is:
muscle
'it' is the definition.
The definition suggests a singular noun which matches the answer.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for muscle that I've seen before include "Beef" , "Contracting body part" , "Might, perhaps" , "elevator, for instance" , "Body tissue that effects movement" .)