Release relative heading off (5)
I believe the answer is:
untie
'release' is the definition.
(I've seen this before)
'relative heading off' is the wordplay.
'relative' becomes 'auntie' (auntie is a kind of relative).
'heading off' means to remove the first letter.
'auntie' with its first letter taken off is 'UNTIE'.
(Other definitions for untie that I've seen before include "Loosen ropes or knots" , "Unite (anag)" , "Undo a knot" , "Undo, unlace" , "Undo, unfasten" .)