Relative takes off top to loosen up (5)
I believe the answer is:
untie
'loosen up' is the definition.
(loosen ties)
'relative takes off top' is the wordplay.
'relative' becomes 'auntie' (auntie is a kind of relative).
'takes off top' means to remove the first letter (I've seen 'taking off top' mean this).
'auntie' with its first letter taken off is 'UNTIE'.
'to' acts as a link.
(Other definitions for untie that I've seen before include "Release bindings or shoelaces" , "Release from bonds or open shoelace" , "Loose bonds" , "Release (knots)" , "Detach" .)