Joint weak when ankle broke, finally (4)
I believe the answer is:
knee
'joint' is the definition.
(I know that knee is a type of hinge joint)
'weak when ankle broke finally' is the wordplay.
'finally' suggests the final letters.
The final letters of 'weak when ankle broke' are 'KNEE'.
(Other definitions for knee that I've seen before include "This is capped by the patella" , "Lower limb joint" , "proposal might be made on one" , "Wounded in Indian Wars action" , "one is bent on submission" .)