Female relative, topless, let loose (5)
I believe the answer is:
'let loose' is the definition.
(I've seen this before)
'female relative topless' is the wordplay.
'female relative' becomes 'auntie' (I've seen this before).
'topless' means to remove the first letter.
'auntie' with its initial letter taken away is 'UNTIE'.
(Other definitions for untie that I've seen before include "Detach", "Open a knot", "Remove fastenings from", "Set free", "Release (knots)".)