Joint legal document protecting son (5)
I believe the answer is:
wrist
'joint' is the definition.
(I've seen this before)
'legal document protecting son' is the wordplay.
'legal document' becomes 'writ' (writ is a kind of legal document**).
'protecting' is an insertion indicator.
'son' becomes 's'.
'writ' going around 's' is 'WRIST'.
(Other definitions for wrist that I've seen before include "Where to keep watch" , "Joint between hand and arm" , "Arm to hand connection, the carpus" , "Joint (the carpus)" , "Human joint" .)