Pain in some strained body part (7)
I believe the answer is:
trachea
'body part' is the definition.
('trachea' is part of 'body')
'pain in some strained' is the wordplay.
'pain' becomes 'ache' (ache is a kind of pain).
'in' indicates putting letters inside.
'some strained' becomes 'tra' (I can't justify this - if you can you should give a lot more credence to this answer).
'ache' put inside 'tra' is 'TRACHEA'.
Can you help me to learn more?
(Other definitions for trachea that I've seen before include "The formal windpipe" , "air passage" , "membranous tube" , "get air supply through this" , "pipe" .)