Drug courier takes on lawyer - heavy! (6)
I believe the answer is:
muscle
'drug' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'courier takes on lawyer heavy' is the wordplay.
'courier' becomes 'mule' (mule can informally mean a courier of drugs).
'takes on' is an insertion indicator.
'lawyer heavy' becomes 'sc' (I can't justify this - if you can you should believe this answer much more).
'mule' placed around 'sc' is 'MUSCLE'.
Can you help me to learn more?
(Other definitions for muscle that I've seen before include "Fibrous tissue with the ability to contract" , "A contractile organ of the body" , "Might, perhaps" , "Strength - to move limbs, etc" , "meat" .)