Argon affected body part (5)
I believe the answer is:
organ
'body part' is the definition.
(organ is a kind of body part)
'argon affected' is the wordplay.
'affected' indicates anagramming the letters.
'argon' with letters rearranged gives 'ORGAN'.
(Other definitions for organ that I've seen before include "Air-powered instrument" , "What makes use of full stops" , "Stopped instrument" , "Church keyboard instrument" , "Large musical instrument often heard in church" .)