Lean on body part (6)
I believe the answer is:
tendon
'body part' is the definition.
(tendon is a kind of body part)
'lean on' is the wordplay.
'lean' becomes 'tend' (synonyms).
'tend'+'on'='TENDON'
(Other definitions for tendon that I've seen before include "Connective tissue" , "Tissue connecting a muscle with its bony attachment" , "Strong cord of bodily tissue" , "personal connecting link" , "Tissue cord connecting muscle to bone" .)