Transgression you said, involves body part? (5)
I believe the answer is:
sinew
'body part?' is the definition.
(I know that sinew is a body part)
'transgression you said involves' is the wordplay.
I cannot quite understand how this works, but
'transgression' could be 'sin' (sin is a kind of transgression) and 'sin' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sinew that I've seen before include "Muscular strength" , "It indicates strength" , "Inelastic tissue" , "Tendon - ligament" , "Tendon, for example" .)