Relative of American set free (6)
I believe the answer is:
Here is my best explanation:
'relative' is the definition.
(auntie is a kind of relative)
'american set free' is the subsidiary indication.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'set free' becomes 'untie' (I've seen this before).
'of' is the link.
(Other definitions for auntie that I've seen before include "Kinswoman", "Relative (colloq.)", "But is she a relative", "Female relative", "beeb", "BBC's nickname", "Family relative", "BBC; relative (colloq,)", "BBC nickname" and "Close relative".)