Release Dad’s sister who lost her head (5)
I believe the answer is:
untie
'release' is the definition.
(eg to untie restraints)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for untie that I've seen before include "Disconnect; release" , "Unite (anag)" , "Release bindings or shoelaces" , "Release, loosen" , "Set free" .)