Father back into the shackle (7)
I believe the answer is:
pastern
'the shackle' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'father back' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'back' becomes 'stern' (stern means the back of a boat).
'pa'+'stern'='PASTERN'
'into' is the link.
Can you help me to learn more?
(Other definitions for pastern that I've seen before include "Wrongly-defined part of horse" , "Word Johnson misdefined" , "part of Arab's leg" , "Fetter-bone of horse" , "Part of horse's foot" .)