Release relative missing the start (5)
I believe the answer is:
'release' is the definition.
(eg to untie restraints)
'relative missing the start' is the wordplay.
'relative' becomes 'auntie' (auntie is a kind of relative).
'missing the start' means to remove the first letter.
'auntie' with its first letter removed is 'UNTIE'.
(Other definitions for untie that I've seen before include "Set free" , "Free from binds" , "Release from bonds or open shoelace" , "Separate" , "Open a knot" .)