One shielding joint quietly had knocked back a couple of lines (7)
I believe the answer is:
patella
'one shielding joint' is the definition.
'patella' can be an answer for 'joint' (I've seen this before). I'm not sure about the 'one shielding' bit.
'quietly had knocked back a couple of lines' is the wordplay.
'quietly' becomes 'p' (musical symbol).
'had' becomes 'ate' ('have' can mean 'eat').
'knocked back' shows that the letters should be reversed in order.
'couple of lines' becomes 'll' (two abbreviations for line).
'a'+'ll'='all'
'all' backwards is 'lla'.
'p'+'ate'+'lla'='PATELLA'
Can you help me to learn more?
(Other definitions for patella that I've seen before include "showing a bit of leg" , "Knee bone" , "bit of body usually under trousers!" , "The formal kneecap" , "positioned on each knee" .)